bpf: Reject non-scalar bpf_loop iteration counts

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bpf_loop() declares its nr_loops argument as ARG_ANYTHING. Privileged
programs may pass pointer values to such arguments, so check_func_arg()
lets a pointer-valued R1 reach the helper-specific checks.

Since commit bb124da69c ("bpf: keep track of max number of bpf_loop
callback iterations"), the verifier marks R1 precise and reads its upper
bound to limit callback simulation. Precision backtracking only accepts
scalar registers, so passing a pointer instead triggers the "backtracking
misuse" verifier warning. Kernels with panic_on_warn enabled subsequently
panic.

Introduce ARG_SCALAR for helper arguments that only accept scalar values
and use it for bpf_loop() nr_loops. Generic helper argument validation then
rejects pointers before loop inlining and precision processing.

Fixes: bb124da69c ("bpf: keep track of max number of bpf_loop callback iterations")
Reported-by: syzbot+7b47f87674e9a1569110@syzkaller.appspotmail.com
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://patch.msgid.link/20260905014735.1452988-2-memxor@gmail.com
Closes: https://lore.kernel.org/bpf/6a9ad24c.b5d4176b.238c3e.0001.GAE@google.com/
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>

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This commit is contained in:

Kumar Kartikeya Dwivedi 2026-09-05 03:47:33 +02:00 • committed by Eduard Zingerman

parent b75a000f2a

commit c3fd8e5fd1

3 changed files with 3 additions and 1 deletions

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1

include/linux/bpf.h

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2

kernel/bpf/bpf_iter.c

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1

kernel/bpf/verifier.c

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