[PATCH bpf-next v3 0/2] bpf, verifier: fix PTR_TO_FLOW_KEYS constant-offset OOB

[PATCH bpf-next v3 0/2] bpf, verifier: fix PTR_TO_FLOW_KEYS constant-offset OOB

[Nuoqi Gui]

A constant offset added to a PTR_TO_FLOW_KEYS register lands in
reg->var_off, but check_flow_keys_access() bounds-checks only insn->off
and never folds reg->var_off.value.  A BPF_PROG_TYPE_FLOW_DISSECTOR
program can therefore do "flow_keys += 0x1000; *(flow_keys + 0)" and have
it accepted, then read/write kernel stack past struct bpf_flow_keys at
runtime.  Patch 1 folds reg->var_off.value into the offset (and rejects
non-constant offsets), mirroring check_ctx_access(); patch 2 adds verifier
selftests.

This is a regression introduced in the 7.1 development cycle by commit
022ac0750883 ("bpf: use reg->var_off instead of reg->off for pointers"),
which moved the constant offset from reg->off (folded generically before
022ac0750883) into reg->var_off without updating the flow_keys path.  No
released kernel is affected: v7.0.x rejects the program above, and the bug
reproduces only on v7.1-rc1..rc5, so no stable backport is needed.

It was first reported privately to security@kernel.org; per their guidance
it is handled in the open as a normal regression fix.  Found by manual
verifier audit and confirmed dynamically in a disposable QEMU/KVM guest:
the load above is accepted, a runtime read leaked a kernel-stack pointer
0x1000 past bpf_flow_keys, and a runtime write of a marker faulted the
guest in net_rx_action.

An alternative -- forbidding pointer arithmetic on PTR_TO_FLOW_KEYS
outright by dropping "if (known) break;" in adjust_ptr_min_max_vals() --
was rejected because v7.0.x accepted (and correctly bounds-checked)
constant arithmetic on the keys pointer; restoring the fold preserves that
behaviour while closing the divergence.

Signed-off-by: Nuoqi Gui <gnq25@mails.tsinghua.edu.cn>
---
v2 -> v3:
 - Pass existing reg/argno context into check_flow_keys_access(), avoiding
   a stale regno reference in check_mem_access().
 - Add a variable-offset selftest using bpf_get_prandom_u32().

v1 -> v2:
 - Target bpf-next instead of bpf (per reviewer feedback).
 - Base-commit updated to bpf-next/master.

v2: https://lore.kernel.org/bpf/20260604180730.2518088-1-gnq25@mails.tsinghua.edu.cn/
v1: https://lore.kernel.org/bpf/20260604150755.2487555-1-gnq25@mails.tsinghua.edu.cn/

To: Alexei Starovoitov <ast@kernel.org>
To: Daniel Borkmann <daniel@iogearbox.net>
To: Andrii Nakryiko <andrii@kernel.org>
To: Eduard Zingerman <eddyz87@gmail.com>
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Cc: Martin KaFai Lau <martin.lau@linux.dev>
Cc: Song Liu <song@kernel.org>
Cc: Yonghong Song <yonghong.song@linux.dev>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Emil Tsalapatis <emil@etsalapatis.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: bpf@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-kselftest@vger.kernel.org

---
Nuoqi Gui (2):
      bpf, verifier: fold reg->var_off into PTR_TO_FLOW_KEYS bounds check
      selftests/bpf: add tests for PTR_TO_FLOW_KEYS offset bounds

 kernel/bpf/verifier.c                              | 18 +++-
 tools/testing/selftests/bpf/prog_tests/verifier.c  |  2 +
 .../selftests/bpf/progs/verifier_flow_keys.c       | 97 ++++++++++++++++++++++
 3 files changed, 114 insertions(+), 3 deletions(-)
---
base-commit: 8ddce416797b7454ba1df855821b02c6e43b5a0e
change-id: 20260606-c3-01-v3-28c680464a7c

Best regards,
--  
Nuoqi Gui <gnq25@mails.tsinghua.edu.cn>

[PATCH bpf-next v3 1/2] bpf, verifier: fold reg->var_off into PTR_TO_FLOW_KEYS bounds check

[Nuoqi Gui]

Constant pointer arithmetic on a PTR_TO_FLOW_KEYS register lands the
constant in reg->var_off (e.g. flow_keys(imm=4096)), but the
PTR_TO_FLOW_KEYS path in check_mem_access() passes only insn->off to
check_flow_keys_access() and never folds reg->var_off.value.  The
verifier therefore accepts an access that, at runtime, dereferences past
struct bpf_flow_keys -- a verifier/runtime divergence that yields an
out-of-bounds read and write of kernel stack memory.

Commit 022ac0750883 ("bpf: use reg->var_off instead of reg->off for
pointers") removed the generic "off += reg->off" that check_mem_access()
applied before the per-type dispatch and replaced it with per-path
folding of reg->var_off.value (for example the ctx path now folds the
register offset via check_ctx_access()).  The PTR_TO_FLOW_KEYS path was
not given the equivalent fold, so a constant offset that used to be
folded and rejected is now silently accepted:

  before 022ac0750883: the offset stays in reg->off and is folded
    generically, so the access is checked with off=4096 and rejected.
  after  022ac0750883: the offset lands in reg->var_off, the flow_keys
    path checks off=0 and accepts; at runtime the access dereferences
    base + 0x1000.

For a BPF_PROG_TYPE_FLOW_DISSECTOR program the following is accepted:

  r2 = *(u64 *)(r1 + 144)   ; R2=flow_keys (PTR_TO_FLOW_KEYS)
  r2 += 0x1000              ; R2=flow_keys(imm=4096), accepted
  r0 = *(u64 *)(r2 + 0)     ; accepted, var_off.value=0x1000 ignored

while the equivalent insn->off form

  r0 = *(u64 *)(r2 + 0x1000)

has the same effective offset but is correctly rejected with
"invalid access to flow keys off=4096 size=8", which isolates the defect
to the missing var_off fold.  Once attached as a flow dissector, the
accepted program reads kernel stack past struct bpf_flow_keys (a
kernel-stack / KASLR information leak) and can likewise write past it,
corrupting kernel memory.

Fix it by folding reg->var_off.value into the offset before the bounds
check and rejecting non-constant offsets, mirroring the other pointer
types (e.g. check_ctx_access()).

No released kernel is affected; the regression is confined to the 7.1
development cycle (reproduced on v7.1-rc1..rc5), and v7.0.x rejects the
program above.

Fixes: 022ac0750883 ("bpf: use reg->var_off instead of reg->off for pointers")
Signed-off-by: Nuoqi Gui <gnq25@mails.tsinghua.edu.cn>
---
 kernel/bpf/verifier.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 935595138aa0..68ddd465584c 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -4728,9 +4728,21 @@ static int check_ctx_access(struct bpf_verifier_env *env, int insn_idx, struct b
 	return err;
 }
 
-static int check_flow_keys_access(struct bpf_verifier_env *env, int off,
-				  int size)
+static int check_flow_keys_access(struct bpf_verifier_env *env,
+				  struct bpf_reg_state *reg, argno_t argno,
+				  int off, int size)
 {
+	/* Only a constant offset is allowed here; fold it into off. */
+	if (!tnum_is_const(reg->var_off)) {
+		char tn_buf[48];
+
+		tnum_strn(tn_buf, sizeof(tn_buf), reg->var_off);
+		verbose(env, "%s invalid variable offset to flow keys: off=%d, var_off=%s\n",
+			reg_arg_name(env, argno), off, tn_buf);
+		return -EACCES;
+	}
+	off += reg->var_off.value;
+
 	if (size < 0 || off < 0 ||
 	    (u64)off + size > sizeof(struct bpf_flow_keys)) {
 		verbose(env, "invalid access to flow keys off=%d size=%d\n",
@@ -6239,7 +6251,7 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, struct b
 			return -EACCES;
 		}
 
-		err = check_flow_keys_access(env, off, size);
+		err = check_flow_keys_access(env, reg, argno, off, size);
 		if (!err && t == BPF_READ && value_regno >= 0)
 			mark_reg_unknown(env, regs, value_regno);
 	} else if (type_is_sk_pointer(reg->type)) {

-- 
2.34.1

Re: [PATCH bpf-next v3 1/2] bpf, verifier: fold reg->var_off into PTR_TO_FLOW_KEYS bounds check

[Eduard Zingerman]

On Sat, 2026-06-06 at 18:50 +0800, Nuoqi Gui wrote:
> Constant pointer arithmetic on a PTR_TO_FLOW_KEYS register lands the
> constant in reg->var_off (e.g. flow_keys(imm=4096)), but the
> PTR_TO_FLOW_KEYS path in check_mem_access() passes only insn->off to
> check_flow_keys_access() and never folds reg->var_off.value.  The
> verifier therefore accepts an access that, at runtime, dereferences past
> struct bpf_flow_keys -- a verifier/runtime divergence that yields an
> out-of-bounds read and write of kernel stack memory.
> 
> Commit 022ac0750883 ("bpf: use reg->var_off instead of reg->off for
> pointers") removed the generic "off += reg->off" that check_mem_access()
> applied before the per-type dispatch and replaced it with per-path
> folding of reg->var_off.value (for example the ctx path now folds the
> register offset via check_ctx_access()).  The PTR_TO_FLOW_KEYS path was
> not given the equivalent fold, so a constant offset that used to be
> folded and rejected is now silently accepted:
> 
>   before 022ac0750883: the offset stays in reg->off and is folded
>     generically, so the access is checked with off=4096 and rejected.
>   after  022ac0750883: the offset lands in reg->var_off, the flow_keys
>     path checks off=0 and accepts; at runtime the access dereferences
>     base + 0x1000.
> 
> For a BPF_PROG_TYPE_FLOW_DISSECTOR program the following is accepted:
> 
>   r2 = *(u64 *)(r1 + 144)   ; R2=flow_keys (PTR_TO_FLOW_KEYS)
>   r2 += 0x1000              ; R2=flow_keys(imm=4096), accepted
>   r0 = *(u64 *)(r2 + 0)     ; accepted, var_off.value=0x1000 ignored
> 
> while the equivalent insn->off form
> 
>   r0 = *(u64 *)(r2 + 0x1000)
> 
> has the same effective offset but is correctly rejected with
> "invalid access to flow keys off=4096 size=8", which isolates the defect
> to the missing var_off fold.  Once attached as a flow dissector, the
> accepted program reads kernel stack past struct bpf_flow_keys (a
> kernel-stack / KASLR information leak) and can likewise write past it,
> corrupting kernel memory.
> 
> Fix it by folding reg->var_off.value into the offset before the bounds
> check and rejecting non-constant offsets, mirroring the other pointer
> types (e.g. check_ctx_access()).
> 
> No released kernel is affected; the regression is confined to the 7.1
> development cycle (reproduced on v7.1-rc1..rc5), and v7.0.x rejects the
> program above.
> 
> Fixes: 022ac0750883 ("bpf: use reg->var_off instead of reg->off for pointers")
> Signed-off-by: Nuoqi Gui <gnq25@mails.tsinghua.edu.cn>
> ---

Acked-by: Eduard Zingerman <eddyz87@gmail.com>

[...]

[PATCH bpf-next v3 2/2] selftests/bpf: add tests for PTR_TO_FLOW_KEYS offset bounds

[Nuoqi Gui]

Add verifier tests covering pointer arithmetic on a PTR_TO_FLOW_KEYS
register. This covers the bpf-next regression where an out-of-bounds
constant offset introduced as flow_keys += K and then dereferenced at
insn->off 0 was accepted, while the equivalent flow_keys + K direct offset
was rejected.

The tests check that in-bounds constant arithmetic on the keys pointer is
still accepted, out-of-bounds constant arithmetic is rejected for both read
and write, and a truly varying offset from bpf_get_prandom_u32() remains
rejected by the existing PTR_TO_FLOW_KEYS pointer arithmetic rules.

Signed-off-by: Nuoqi Gui <gnq25@mails.tsinghua.edu.cn>
---
 tools/testing/selftests/bpf/prog_tests/verifier.c  |  2 +
 .../selftests/bpf/progs/verifier_flow_keys.c       | 97 ++++++++++++++++++++++
 2 files changed, 99 insertions(+)

diff --git a/tools/testing/selftests/bpf/prog_tests/verifier.c b/tools/testing/selftests/bpf/prog_tests/verifier.c
index 89779d897aba..8a3d69e2453c 100644
--- a/tools/testing/selftests/bpf/prog_tests/verifier.c
+++ b/tools/testing/selftests/bpf/prog_tests/verifier.c
@@ -38,6 +38,7 @@
 #include "verifier_div0.skel.h"
 #include "verifier_div_mod_bounds.skel.h"
 #include "verifier_div_overflow.skel.h"
+#include "verifier_flow_keys.skel.h"
 #include "verifier_global_subprogs.skel.h"
 #include "verifier_global_ptr_args.skel.h"
 #include "verifier_gotol.skel.h"
@@ -190,6 +191,7 @@ void test_verifier_direct_stack_access_wraparound(void) { RUN(verifier_direct_st
 void test_verifier_div0(void)                 { RUN(verifier_div0); }
 void test_verifier_div_mod_bounds(void)       { RUN(verifier_div_mod_bounds); }
 void test_verifier_div_overflow(void)         { RUN(verifier_div_overflow); }
+void test_verifier_flow_keys(void)            { RUN(verifier_flow_keys); }
 void test_verifier_global_subprogs(void)      { RUN(verifier_global_subprogs); }
 void test_verifier_global_ptr_args(void)      { RUN(verifier_global_ptr_args); }
 void test_verifier_gotol(void)                { RUN(verifier_gotol); }
diff --git a/tools/testing/selftests/bpf/progs/verifier_flow_keys.c b/tools/testing/selftests/bpf/progs/verifier_flow_keys.c
new file mode 100644
index 000000000000..d780a36a6e9a
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/verifier_flow_keys.c
@@ -0,0 +1,97 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Bounds checks for PTR_TO_FLOW_KEYS pointer arithmetic. */
+
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include "bpf_misc.h"
+
+/* sizeof(struct bpf_flow_keys) is well under 4096, so +0x1000 is OOB. */
+
+SEC("flow_dissector")
+__description("flow_keys: in-bounds constant pointer arithmetic accepted")
+__success
+__naked void flow_keys_const_inbounds(void)
+{
+	asm volatile ("					\
+	r1 = *(u64 *)(r1 + %[flow_keys]);		\
+	r1 += 8;					\
+	r0 = *(u64 *)(r1 + 0);				\
+	r0 = 0;						\
+	exit;						\
+"	:
+	: __imm_const(flow_keys, offsetof(struct __sk_buff, flow_keys))
+	: __clobber_all);
+}
+
+SEC("flow_dissector")
+__description("flow_keys: OOB via constant pointer arithmetic rejected")
+__failure __msg("invalid access to flow keys off=4096 size=8")
+__naked void flow_keys_const_oob_read(void)
+{
+	asm volatile ("					\
+	r1 = *(u64 *)(r1 + %[flow_keys]);		\
+	r1 += 4096;					\
+	r0 = *(u64 *)(r1 + 0);				\
+	r0 = 0;						\
+	exit;						\
+"	:
+	: __imm_const(flow_keys, offsetof(struct __sk_buff, flow_keys))
+	: __clobber_all);
+}
+
+SEC("flow_dissector")
+__description("flow_keys: OOB write via constant pointer arithmetic rejected")
+__failure __msg("invalid access to flow keys off=4096 size=8")
+__naked void flow_keys_const_oob_write(void)
+{
+	asm volatile ("					\
+	r1 = *(u64 *)(r1 + %[flow_keys]);		\
+	r1 += 4096;					\
+	r2 = 0;						\
+	*(u64 *)(r1 + 0) = r2;				\
+	r0 = 0;						\
+	exit;						\
+"	:
+	: __imm_const(flow_keys, offsetof(struct __sk_buff, flow_keys))
+	: __clobber_all);
+}
+
+/* Equivalent OOB expressed directly in insn->off; this form was always
+ * rejected and is kept to show both forms now share one diagnostic.
+ */
+SEC("flow_dissector")
+__description("flow_keys: OOB via insn->off rejected")
+__failure __msg("invalid access to flow keys off=4096 size=8")
+__naked void flow_keys_insn_off_oob(void)
+{
+	asm volatile ("					\
+	r1 = *(u64 *)(r1 + %[flow_keys]);		\
+	r0 = *(u64 *)(r1 + 4096);			\
+	r0 = 0;						\
+	exit;						\
+"	:
+	: __imm_const(flow_keys, offsetof(struct __sk_buff, flow_keys))
+	: __clobber_all);
+}
+
+SEC("flow_dissector")
+__description("flow_keys: variable pointer arithmetic rejected")
+__failure __msg("R1 pointer arithmetic on flow_keys prohibited")
+__naked void flow_keys_var_read(void)
+{
+	asm volatile ("					\
+	r6 = r1;					\
+	call %[bpf_get_prandom_u32];			\
+	r0 &= 0xFFFF;					\
+	r1 = *(u64 *)(r6 + %[flow_keys]);		\
+	r1 += r0;					\
+	r0 = *(u64 *)(r1 + 0);				\
+	r0 = 0;						\
+	exit;						\
+"	:
+	: __imm_const(flow_keys, offsetof(struct __sk_buff, flow_keys)),
+	  __imm(bpf_get_prandom_u32)
+	: __clobber_all);
+}
+
+char _license[] SEC("license") = "GPL";

-- 
2.34.1

Re: [PATCH bpf-next v3 2/2] selftests/bpf: add tests for PTR_TO_FLOW_KEYS offset bounds

[bot+bpf-ci]

[-- Attachment #1: Type: text/plain, Size: 1693 bytes --]

> diff --git a/tools/testing/selftests/bpf/prog_tests/verifier.c b/tools/testing/selftests/bpf/prog_tests/verifier.c
> index 89779d897aba..8a3d69e2453c 100644
> --- a/tools/testing/selftests/bpf/prog_tests/verifier.c
> +++ b/tools/testing/selftests/bpf/prog_tests/verifier.c

[ ... ]

> diff --git a/tools/testing/selftests/bpf/progs/verifier_flow_keys.c b/tools/testing/selftests/bpf/progs/verifier_flow_keys.c
> new file mode 100644
> index 000000000000..d780a36a6e9a
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/verifier_flow_keys.c
> @@ -0,0 +1,97 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Bounds checks for PTR_TO_FLOW_KEYS pointer arithmetic. */

[ ... ]

> +
> +/* Equivalent OOB expressed directly in insn->off; this form was always
> + * rejected and is kept to show both forms now share one diagnostic.
> + */

This isn't a bug, but does this multi-line comment follow the preferred
BPF subsystem style? The guidelines specify that the opening /* should be
on its own line, with the comment text starting on the next line.

Note: This style issue has been consistently raised in reviews for v1, v2,
and v3 but has not been addressed.

Reference: https://lore.kernel.org/bpf/20260604154507.B5CB51F00893@smtp.kernel.org/

> +SEC("flow_dissector")
> +__description("flow_keys: OOB via insn->off rejected")
> +__failure __msg("invalid access to flow keys off=4096 size=8")
> +__naked void flow_keys_insn_off_oob(void)

[ ... ]


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/27060463888

Re: [PATCH bpf-next v3 2/2] selftests/bpf: add tests for PTR_TO_FLOW_KEYS offset bounds

[Eduard Zingerman]

On Sat, 2026-06-06 at 18:50 +0800, Nuoqi Gui wrote:
> Add verifier tests covering pointer arithmetic on a PTR_TO_FLOW_KEYS
> register. This covers the bpf-next regression where an out-of-bounds
> constant offset introduced as flow_keys += K and then dereferenced at
> insn->off 0 was accepted, while the equivalent flow_keys + K direct offset
> was rejected.
> 
> The tests check that in-bounds constant arithmetic on the keys pointer is
> still accepted, out-of-bounds constant arithmetic is rejected for both read
> and write, and a truly varying offset from bpf_get_prandom_u32() remains
> rejected by the existing PTR_TO_FLOW_KEYS pointer arithmetic rules.
> 
> Signed-off-by: Nuoqi Gui <gnq25@mails.tsinghua.edu.cn>
> ---

Acked-by: Eduard Zingerman <eddyz87@gmail.com>

[...]

Re: [PATCH bpf-next v3 0/2] bpf, verifier: fix PTR_TO_FLOW_KEYS constant-offset OOB

[patchwork-bot+netdevbpf]

Hello:

This series was applied to bpf/bpf-next.git (master)
by Alexei Starovoitov <ast@kernel.org>:

On Sat, 06 Jun 2026 18:50:36 +0800 you wrote:
> A constant offset added to a PTR_TO_FLOW_KEYS register lands in
> reg->var_off, but check_flow_keys_access() bounds-checks only insn->off
> and never folds reg->var_off.value.  A BPF_PROG_TYPE_FLOW_DISSECTOR
> program can therefore do "flow_keys += 0x1000; *(flow_keys + 0)" and have
> it accepted, then read/write kernel stack past struct bpf_flow_keys at
> runtime.  Patch 1 folds reg->var_off.value into the offset (and rejects
> non-constant offsets), mirroring check_ctx_access(); patch 2 adds verifier
> selftests.
> 
> [...]

Here is the summary with links:
  - [bpf-next,v3,1/2] bpf, verifier: fold reg->var_off into PTR_TO_FLOW_KEYS bounds check
    https://git.kernel.org/bpf/bpf-next/c/37363191cbe8
  - [bpf-next,v3,2/2] selftests/bpf: add tests for PTR_TO_FLOW_KEYS offset bounds
    https://git.kernel.org/bpf/bpf-next/c/3ce6b42458f0

You are awesome, thank you!
-- 
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