[PATCH bpf-next v2] bpf: Use memmove() to load bytes from an skb and an xdp_buff

[Ayushman Rout <ayushmanrout27@gmail.com>]

bpf_dynptr_read() on an skb or xdp dynptr ends in bpf_skb_load_bytes()
or bpf_xdp_load_bytes(). The destination can be a slice of the same
packet, so it can overlap the source, which memcpy() does not allow.
On x86 with FSRM memcpy() copies forward, so a destination above the
source gets the first bytes repeated:

  p = bpf_dynptr_slice_rdwr(&ptr, 8, NULL, 64);
  bpf_dynptr_read(p, 64, &ptr, 0, 0);

leaves bytes 8-71 of the packet as bytes 0-7 repeated, not bytes 0-63.
Reproduced with BPF_PROG_TEST_RUN on both an xdp and a tc program.

Use memmove(), as the store side does since commit 1db084f42884
("bpf: Use memmove() to store bytes in an skb") and commit ab08863c58b1
("bpf: Use memmove() to store bytes in an xdp_buff"). A read that spans
xdp frags still goes through bpf_xdp_copy_buf() and is not changed
here.

bpf_skb_load_bytes_relative() has the same pattern and is reachable
from tc and sk_filter programs, so use memmove() there as well.
Reproduced with BPF_PROG_TEST_RUN on a tc program: loading into a
slice at offset 8 gave wrong bytes before the change and correct
bytes after.

Fixes: 2012c867c800 ("bpf: verifier: Accept dynptr mem as mem in helpers")
Signed-off-by: Ayushman Rout <ayushmanrout27@gmail.com>
Assisted-by: LLM
---
v1: https://lore.kernel.org/bpf/20261021103957.17532-1-ayushmanrout27@gmail.com/
v1 -> v2: also use memmove() in bpf_skb_load_bytes_relative() (review by bot and Alexei)

 net/core/filter.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index b7ac145b68b1..09c60a58c82a 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -1789,7 +1789,7 @@ BPF_CALL_4(bpf_skb_load_bytes, const struct sk_buff *, skb, u32, offset,
 	if (unlikely(!ptr))
 		goto err_clear;
 	if (ptr != to)
-		memcpy(to, ptr, len);
+		memmove(to, ptr, len);
 
 	return 0;
 err_clear:
@@ -1871,7 +1871,7 @@ BPF_CALL_5(bpf_skb_load_bytes_relative, const struct sk_buff *, skb,
 	ptr = start + offset;
 
 	if (likely(ptr + len <= end)) {
-		memcpy(to, ptr, len);
+		memmove(to, ptr, len);
 		return 0;
 	}
 
@@ -4298,7 +4298,7 @@ BPF_CALL_4(bpf_xdp_load_bytes, struct xdp_buff *, xdp, u32, offset,
 	if (!ptr)
 		bpf_xdp_copy_buf(xdp, offset, buf, len, false);
 	else
-		memcpy(buf, ptr, len);
+		memmove(buf, ptr, len);
 
 	return 0;
 }
-- 
2.54.0