[PATCH stable 6.10 1/2] bpf: Fix a kernel verifier crash in stacksafe()

[Shung-Hsi Yu <shung-hsi.yu@suse.com>]

From: Yonghong Song <yonghong.song@linux.dev>

[ Upstream commit bed2eb964c70b780fb55925892a74f26cb590b25 ]

Daniel Hodges reported a kernel verifier crash when playing with sched-ext.
Further investigation shows that the crash is due to invalid memory access
in stacksafe(). More specifically, it is the following code:

    if (exact != NOT_EXACT &&
        old->stack[spi].slot_type[i % BPF_REG_SIZE] !=
        cur->stack[spi].slot_type[i % BPF_REG_SIZE])
            return false;

The 'i' iterates old->allocated_stack.
If cur->allocated_stack < old->allocated_stack the out-of-bound
access will happen.

To fix the issue add 'i >= cur->allocated_stack' check such that if
the condition is true, stacksafe() should fail. Otherwise,
cur->stack[spi].slot_type[i % BPF_REG_SIZE] memory access is legal.

Fixes: 2793a8b015f7 ("bpf: exact states comparison for iterator convergence checks")
Cc: Eduard Zingerman <eddyz87@gmail.com>
Reported-by: Daniel Hodges <hodgesd@meta.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20240812214847.213612-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
---
I see this patch itself was already picked up[1] by Sasha. This thread
additional includes the associated selftest that was sent in the same
series (not entirely sure if backporting selftest goes against the
stable backporting rule though).

1: https://git.kernel.org/pub/scm/linux/kernel/git/stable/stable-queue.git/commit/queue-6.10/bpf-fix-a-kernel-verifier-crash-in-stacksafe.patch?id=e1b8b4a2ab5985f048c5e4ba7575deaa7bb65df7
---
 kernel/bpf/verifier.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index a8845cc299fe..521bd7efae03 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -16881,8 +16881,9 @@ static bool stacksafe(struct bpf_verifier_env *env, struct bpf_func_state *old,
 		spi = i / BPF_REG_SIZE;
 
 		if (exact != NOT_EXACT &&
-		    old->stack[spi].slot_type[i % BPF_REG_SIZE] !=
-		    cur->stack[spi].slot_type[i % BPF_REG_SIZE])
+		    (i >= cur->allocated_stack ||
+		     old->stack[spi].slot_type[i % BPF_REG_SIZE] !=
+		     cur->stack[spi].slot_type[i % BPF_REG_SIZE]))
 			return false;
 
 		if (!(old->stack[spi].spilled_ptr.live & REG_LIVE_READ)
-- 
2.46.0