[PATCH bpf-next] bpf: arena: fix mmap_lock deadlock on arena lock failure

[PATCH bpf-next] bpf: arena: fix mmap_lock deadlock on arena lock failure

[Jiayuan Chen]

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: b8467290edab ("bpf: arena: make arena kfuncs any context safe")
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>

---
target to bpf-next since I think it is moderate.
---
 kernel/bpf/arena.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/arena.c b/kernel/bpf/arena.c
index 34f023a537fe..555ee2531ef9 100644
--- a/kernel/bpf/arena.c
+++ b/kernel/bpf/arena.c
@@ -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) {
-- 
2.43.0

Re: [PATCH bpf-next] bpf: arena: fix mmap_lock deadlock on arena lock failure

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] BPF arena page faults can spuriously crash the user process with SIGSEGV due to transient lockless allocation failures.
--

--- Patch [1]: [PATCH bpf-next] bpf: arena: fix mmap_lock deadlock on arena lock failure ---
commit c57623f4077e743e8c901306fcec7c67d2aafc17
Author: Jiayuan Chen <jiayuan.chen@linux.dev>

bpf: arena: fix mmap_lock deadlock on arena lock failure

This commit fixes a potential deadlock in the arena page fault handler by
returning VM_FAULT_SIGBUS instead of VM_FAULT_RETRY when the arena spinlock
cannot be acquired. Returning retry would cause the fault path to attempt
retaking the mmap_lock, which was never dropped.

> diff --git a/kernel/bpf/arena.c b/kernel/bpf/arena.c
> --- a/kernel/bpf/arena.c
> +++ b/kernel/bpf/arena.c
> @@ -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;

[Severity: High]
This is a pre-existing issue, but looking further down in arena_vm_fault(),
does a transient allocation failure unexpectedly cause a SIGSEGV?

Since the raw spinlock is held, disabling IRQs, bpf_map_alloc_pages()
falls back to an opportunistic lockless allocation which can fail under
memory pressure:

	ret = bpf_map_alloc_pages(map, NUMA_NO_NODE, 1, &page);
	if (ret) {
		range_tree_set(&arena->rt, vmf->pgoff, 1);
		goto out_sigsegv_memcg;
	}

When this lockless allocation fails, the error path jumps to
out_sigsegv_memcg and returns VM_FAULT_SIGSEGV instead of VM_FAULT_OOM or
safely dropping the lock to retry.

Could this cause user processes mapping a BPF arena to experience spurious
crashes during memory contention?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260728060517.95183-1-jiayuan.chen@linux.dev?part=1

Re: [PATCH bpf-next] bpf: arena: fix mmap_lock deadlock on arena lock failure

[Jiayuan Chen]

On 7/28/26 2:23 PM, sashiko-bot@kernel.org wrote:
> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
>
> Pre-existing issues:
> - [High] BPF arena page faults can spuriously crash the user process with SIGSEGV due to transient lockless allocation failures.
> --
>
> --- Patch [1]: [PATCH bpf-next] bpf: arena: fix mmap_lock deadlock on arena lock failure ---
> commit c57623f4077e743e8c901306fcec7c67d2aafc17
> Author: Jiayuan Chen <jiayuan.chen@linux.dev>
>
> bpf: arena: fix mmap_lock deadlock on arena lock failure
>
> This commit fixes a potential deadlock in the arena page fault handler by
> returning VM_FAULT_SIGBUS instead of VM_FAULT_RETRY when the arena spinlock
> cannot be acquired. Returning retry would cause the fault path to attempt
> retaking the mmap_lock, which was never dropped.
>
>> diff --git a/kernel/bpf/arena.c b/kernel/bpf/arena.c
>> --- a/kernel/bpf/arena.c
>> +++ b/kernel/bpf/arena.c
>> @@ -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;
> [Severity: High]
> This is a pre-existing issue, but looking further down in arena_vm_fault(),
> does a transient allocation failure unexpectedly cause a SIGSEGV?
>
> Since the raw spinlock is held, disabling IRQs, bpf_map_alloc_pages()
> falls back to an opportunistic lockless allocation which can fail under
> memory pressure:
>
> 	ret = bpf_map_alloc_pages(map, NUMA_NO_NODE, 1, &page);
> 	if (ret) {
> 		range_tree_set(&arena->rt, vmf->pgoff, 1);
> 		goto out_sigsegv_memcg;
> 	}
>
> When this lockless allocation fails, the error path jumps to
> out_sigsegv_memcg and returns VM_FAULT_SIGSEGV instead of VM_FAULT_OOM or
> safely dropping the lock to retry.
>
> Could this cause user processes mapping a BPF arena to experience spurious
> crashes during memory contention?


Yes I'm on it 
https://lore.kernel.org/bpf/20260727062521.376231-1-jiayuan.chen@linux.dev/

And current patch is from your review result of that patch...