* [PATCH 6.12.y 1/2] arm64/fpsimd: signal: Mandate SVE payload for streaming-mode state
2026-03-03 1:50 [PATCH 6.12.y 0/2] Fix patch backport review Rahul Sharma
@ 2026-03-03 1:50 ` Rahul Sharma
2026-03-03 1:50 ` [PATCH 6.12.y 2/2] arm64/fpsimd: signal: Fix restoration of SVE context Rahul Sharma
2026-03-03 13:08 ` [PATCH 6.12.y 0/2] Fix patch backport review Mark Rutland
2 siblings, 0 replies; 4+ messages in thread
From: Rahul Sharma @ 2026-03-03 1:50 UTC (permalink / raw)
To: gregkh, stable
Cc: linux-kernel, Mark Rutland, Catalin Marinas, Marc Zyngier,
Mark Brown, Will Deacon, Rahul Sharma
From: Mark Rutland <mark.rutland@arm.com>
[ Upstream commit b465ace42620970e840c7aeb2c44a6e3b1002fec ]
Non-streaming SVE state may be preserved without an SVE payload, in
which case the SVE context only has a header with VL==0, and all state
can be restored from the FPSIMD context. Streaming SVE state is always
preserved with an SVE payload, where the SVE context header has VL!=0,
and the SVE_SIG_FLAG_SM flag is set.
The kernel never preserves an SVE context where SVE_SIG_FLAG_SM is set
without an SVE payload. However, restore_sve_fpsimd_context() doesn't
forbid restoring such a context, and will handle this case by clearing
PSTATE.SM and restoring the FPSIMD context into non-streaming mode,
which isn't consistent with the SVE_SIG_FLAG_SM flag.
Forbid this case, and mandate an SVE payload when the SVE_SIG_FLAG_SM
flag is set. This avoids an awkward ABI quirk and reduces the risk that
later rework to this code permits configuring a task with PSTATE.SM==1
and fp_type==FP_STATE_FPSIMD.
I've marked this as a fix given that we never intended to support this
case, and we don't want anyone to start relying upon the old behaviour
once we re-enable SME.
Fixes: 85ed24dad290 ("arm64/sme: Implement streaming SVE signal handling")
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20250508132644.1395904-4-mark.rutland@arm.com
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Rahul Sharma <black.hawk@163.com>
---
arch/arm64/kernel/signal.c | 15 +++++++++++++--
1 file changed, 13 insertions(+), 2 deletions(-)
diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
index f7b05267a76b..2ff6af928426 100644
--- a/arch/arm64/kernel/signal.c
+++ b/arch/arm64/kernel/signal.c
@@ -391,6 +391,7 @@ static int restore_sve_fpsimd_context(struct user_ctxs *user)
unsigned int vl, vq;
struct user_fpsimd_state fpsimd;
u16 user_vl, flags;
+ bool sm;
if (user->sve_size < sizeof(*user->sve))
return -EINVAL;
@@ -400,7 +401,8 @@ static int restore_sve_fpsimd_context(struct user_ctxs *user)
if (err)
return err;
- if (flags & SVE_SIG_FLAG_SM) {
+ sm = flags & SVE_SIG_FLAG_SM;
+ if (sm) {
if (!system_supports_sme())
return -EINVAL;
@@ -420,7 +422,16 @@ static int restore_sve_fpsimd_context(struct user_ctxs *user)
if (user_vl != vl)
return -EINVAL;
- if (user->sve_size == sizeof(*user->sve)) {
+ /*
+ * Non-streaming SVE state may be preserved without an SVE payload, in
+ * which case the SVE context only has a header with VL==0, and all
+ * state can be restored from the FPSIMD context.
+ *
+ * Streaming SVE state is always preserved with an SVE payload. For
+ * consistency and robustness, reject restoring streaming SVE state
+ * without an SVE payload.
+ */
+ if (!sm && user->sve_size == sizeof(*user->sve)) {
clear_thread_flag(TIF_SVE);
current->thread.svcr &= ~SVCR_SM_MASK;
current->thread.fp_type = FP_STATE_FPSIMD;
--
2.34.1
^ permalink raw reply [flat|nested] 4+ messages in thread* [PATCH 6.12.y 2/2] arm64/fpsimd: signal: Fix restoration of SVE context
2026-03-03 1:50 [PATCH 6.12.y 0/2] Fix patch backport review Rahul Sharma
2026-03-03 1:50 ` [PATCH 6.12.y 1/2] arm64/fpsimd: signal: Mandate SVE payload for streaming-mode state Rahul Sharma
@ 2026-03-03 1:50 ` Rahul Sharma
2026-03-03 13:08 ` [PATCH 6.12.y 0/2] Fix patch backport review Mark Rutland
2 siblings, 0 replies; 4+ messages in thread
From: Rahul Sharma @ 2026-03-03 1:50 UTC (permalink / raw)
To: gregkh, stable
Cc: linux-kernel, Mark Rutland, Mark Brown, Will Deacon,
Catalin Marinas, Rahul Sharma
From: Mark Rutland <mark.rutland@arm.com>
[ Upstream commit d2907cbe9ea0a54cbe078076f9d089240ee1e2d9 ]
When SME is supported, Restoring SVE signal context can go wrong in a
few ways, including placing the task into an invalid state where the
kernel may read from out-of-bounds memory (and may potentially take a
fatal fault) and/or may kill the task with a SIGKILL.
(1) Restoring a context with SVE_SIG_FLAG_SM set can place the task into
an invalid state where SVCR.SM is set (and sve_state is non-NULL)
but TIF_SME is clear, consequently resuting in out-of-bounds memory
reads and/or killing the task with SIGKILL.
This can only occur in unusual (but legitimate) cases where the SVE
signal context has either been modified by userspace or was saved in
the context of another task (e.g. as with CRIU), as otherwise the
presence of an SVE signal context with SVE_SIG_FLAG_SM implies that
TIF_SME is already set.
While in this state, task_fpsimd_load() will NOT configure SMCR_ELx
(leaving some arbitrary value configured in hardware) before
restoring SVCR and attempting to restore the streaming mode SVE
registers from memory via sve_load_state(). As the value of
SMCR_ELx.LEN may be larger than the task's streaming SVE vector
length, this may read memory outside of the task's allocated
sve_state, reading unrelated data and/or triggering a fault.
While this can result in secrets being loaded into streaming SVE
registers, these values are never exposed. As TIF_SME is clear,
fpsimd_bind_task_to_cpu() will configure CPACR_ELx.SMEN to trap EL0
accesses to streaming mode SVE registers, so these cannot be
accessed directly at EL0. As fpsimd_save_user_state() verifies the
live vector length before saving (S)SVE state to memory, no secret
values can be saved back to memory (and hence cannot be observed via
ptrace, signals, etc).
When the live vector length doesn't match the expected vector length
for the task, fpsimd_save_user_state() will send a fatal SIGKILL
signal to the task. Hence the task may be killed after executing
userspace for some period of time.
(2) Restoring a context with SVE_SIG_FLAG_SM clear does not clear the
task's SVCR.SM. If SVCR.SM was set prior to restoring the context,
then the task will be left in streaming mode unexpectedly, and some
register state will be combined inconsistently, though the task will
be left in legitimate state from the kernel's PoV.
This can only occur in unusual (but legitimate) cases where ptrace
has been used to set SVCR.SM after entry to the sigreturn syscall,
as syscall entry clears SVCR.SM.
In these cases, the the provided SVE register data will be loaded
into the task's sve_state using the non-streaming SVE vector length
and the FPSIMD registers will be merged into this using the
streaming SVE vector length.
Fix (1) by setting TIF_SME when setting SVCR.SM. This also requires
ensuring that the task's sme_state has been allocated, but as this could
contain live ZA state, it should not be zeroed. Fix (2) by clearing
SVCR.SM when restoring a SVE signal context with SVE_SIG_FLAG_SM clear.
For consistency, I've pulled the manipulation of SVCR, TIF_SVE, TIF_SME,
and fp_type earlier, immediately after the allocation of
sve_state/sme_state, before the restore of the actual register state.
This makes it easier to ensure that these are always modified
consistently, even if a fault is taken while reading the register data
from the signal context. I do not expect any software to depend on the
exact state restored when a fault is taken while reading the context.
Fixes: 85ed24dad290 ("arm64/sme: Implement streaming SVE signal handling")
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: <stable@vger.kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
[ The context change is due to the commit 929fa99b1215
("arm64/fpsimd: signal: Always save+flush state early")
and the commit be625d803c3b
("arm64/fpsimd: signal: Consistently read FPSIMD context")
in v6.16 which are irrelevant to the logic of this patch. ]
Signed-off-by: Rahul Sharma <black.hawk@163.com>
---
arch/arm64/kernel/signal.c | 21 +++++++++++++++------
1 file changed, 15 insertions(+), 6 deletions(-)
diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
index 2ff6af928426..65f4e5841ea8 100644
--- a/arch/arm64/kernel/signal.c
+++ b/arch/arm64/kernel/signal.c
@@ -452,6 +452,11 @@ static int restore_sve_fpsimd_context(struct user_ctxs *user)
fpsimd_flush_task_state(current);
/* From now, fpsimd_thread_switch() won't touch thread.sve_state */
+ if (sm) {
+ sme_alloc(current, false);
+ if (!current->thread.sme_state)
+ return -ENOMEM;
+ }
sve_alloc(current, true);
if (!current->thread.sve_state) {
@@ -459,6 +464,16 @@ static int restore_sve_fpsimd_context(struct user_ctxs *user)
return -ENOMEM;
}
+ if (sm) {
+ current->thread.svcr |= SVCR_SM_MASK;
+ set_thread_flag(TIF_SME);
+ } else {
+ current->thread.svcr &= ~SVCR_SM_MASK;
+ set_thread_flag(TIF_SVE);
+ }
+
+ current->thread.fp_type = FP_STATE_SVE;
+
err = __copy_from_user(current->thread.sve_state,
(char __user const *)user->sve +
SVE_SIG_REGS_OFFSET,
@@ -466,12 +481,6 @@ static int restore_sve_fpsimd_context(struct user_ctxs *user)
if (err)
return -EFAULT;
- if (flags & SVE_SIG_FLAG_SM)
- current->thread.svcr |= SVCR_SM_MASK;
- else
- set_thread_flag(TIF_SVE);
- current->thread.fp_type = FP_STATE_SVE;
-
fpsimd_only:
/* copy the FP and status/control registers */
/* restore_sigframe() already checked that user->fpsimd != NULL. */
--
2.34.1
^ permalink raw reply [flat|nested] 4+ messages in thread