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bpf: Fix mmap_lock deadlock on arena lock failure

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Reported by the Sashiko AI review.

arena_vm_fault() returns VM_FAULT_RETRY when it can't take
arena->spinlock, but it never took mmap_lock. The fault path assumes a
VM_FAULT_RETRY handler already dropped mmap_lock and re-takes it on the
retry, so mmap_lock gets taken twice and can deadlock:

	do_user_addr_fault()
	{
		fault = handle_mm_fault(...);   // calls arena_vm_fault()
		if (fault & VM_FAULT_RETRY)
			goto retry;   // re-locks mmap_lock
		mmap_read_unlock(mm);
	}

Return VM_FAULT_SIGBUS instead, for two reasons:

1. We could keep VM_FAULT_RETRY, but then we'd have to drop the fault
   lock first and cap the retry ourselves, the way __folio_lock_or_retry()
   does.

2. A failed raw_res_spin_lock_irqsave() already means a possible deadlock
   was detected, so retrying just hits the same lock again.

So returning VM_FAULT_RETRY here is overkill.

Fixes: b8467290ed ("bpf: arena: make arena kfuncs any context safe")
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Link: https://lore.kernel.org/bpf/20260728060517.95183-1-jiayuan.chen@linux.dev
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
This commit is contained in:
Jiayuan Chen 2026-07-28 14:05:16 +08:00 committed by Kumar Kartikeya Dwivedi
parent 8f876c773b
commit 0b10b94547
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GPG Key ID: 472D377B63542F83
1 changed files with 6 additions and 2 deletions
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8
kernel/bpf/arena.c
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@ -490,8 +490,12 @@ static vm_fault_t arena_vm_fault(struct vm_fault *vmf)
kaddr = kbase + (u32)(vmf->address);
if (raw_res_spin_lock_irqsave(&arena->spinlock, flags))
/* Make a reasonable effort to address impossible case */
return VM_FAULT_RETRY;
/*
* A failed lock means a possible deadlock was detected. Don't
* return VM_FAULT_RETRY: this handler never took mmap_lock, but
* the fault path would re-take it on retry and deadlock. Fail.
*/
return VM_FAULT_SIGBUS;
page = vmalloc_to_page((void *)kaddr);
if (page) {