Re: [syzbot] [PATCH] fix array-index-out-of-bounds in bpf_prog_select_runtime

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Subject: [PATCH] fix array-index-out-of-bounds in bpf_prog_select_runtime
Author: cam.alvarez.i@xxxxxxxxx

#syz test
The error indicates that the verifier is letting through a program with
a stack depth bigger than 512.

This is due to the verifier not checking the stack depth after
instruction rewrites are perfomed. For example, the MAY_GOTO instruction
adds 8 bytes to the stack, which means that if the stack at the moment
was already 512 bytes it would overflow after rewriting the instruction.

The fix involves adding a stack depth check after all instruction
rewrites are performed.

Reported-by: syzbot+d2a2c639d03ac200a4f1@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Camila Alvarez <cam.alvarez.i@xxxxxxxxx>
---
 kernel/bpf/verifier.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 63749ad5ac6b..a9e23b6b8e8f 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -21285,6 +21285,10 @@ int bpf_check(struct bpf_prog **prog, union bpf_attr *attr, bpfptr_t uattr, __u3
 	if (ret == 0)
 		ret = do_misc_fixups(env);
 
+        /* max stack depth verification must be done after rewrites as well */
+        if (ret == 0)
+                ret = check_max_stack_depth(env);
+
 	/* do 32-bit optimization after insn patching has done so those patched
 	 * insns could be handled correctly.
 	 */
-- 
2.34.1