Re: [PATCH v1] tracing/remotes: Fix page_va[] access before counter update in trace_remote_alloc_buffer()

[Vincent Donnefort <vdonnefort@google.com>]

On Mon, Jul 13, 2026 at 08:28:23AM +0100, Fuad Tabba wrote:
> page_va[] is annotated __counted_by(nr_page_va), so nr_page_va must
> cover an index before that element is accessed. The allocation loop
> writes page_va[id] while nr_page_va is still id and increments it only
> afterwards, so every write is one element past the declared count.
> 
> The store is out of bounds with respect to the annotation: a build with
> CONFIG_UBSAN_BOUNDS on a toolchain that honours __counted_by
> (clang >= 20.1, gcc >= 15.1) flags it as an array-index overflow.
> 
> Increment nr_page_va before writing the element it now covers. A failed
> allocation then leaves the slot counted but NULL; the error path frees
> it with free_page(0), which is a no-op.
> 
> Fixes: 96e43537af546 ("tracing: Introduce trace remotes")
> Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>

Reviewed-by: Vincent Donnefort <vdonnefort@google.com>
Tested-by: Vincent Donnefort <vdonnefort@google.com>

> ---
> I found this while reviewing Vincent's series [1].
> 
> Applies on top of Vincent's "[PATCH v2 0/3] tracing/remotes: Fix leak in
> trace_remote_alloc_buffer() error path" [2], which fixes the other
> issues in this function.
> 
> [1] https://lore.kernel.org/all/20260710114819.2689386-1-vdonnefort@google.com/
> [2] https://lore.kernel.org/all/20260709160017.1729517-1-vdonnefort@google.com/
> 
>  kernel/trace/trace_remote.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/kernel/trace/trace_remote.c b/kernel/trace/trace_remote.c
> index 0f6ef5c36d84e..ef42d9c38b374 100644
> --- a/kernel/trace/trace_remote.c
> +++ b/kernel/trace/trace_remote.c
> @@ -1004,11 +1004,10 @@ int trace_remote_alloc_buffer(struct trace_buffer_desc *desc, size_t desc_size,
>  		desc->nr_cpus++;
>  
>  		for (id = 0; id < nr_pages; id++) {
> +			rb_desc->nr_page_va++;
>  			rb_desc->page_va[id] = (unsigned long)__get_free_page(GFP_KERNEL);
>  			if (!rb_desc->page_va[id])
>  				goto err;
> -
> -			rb_desc->nr_page_va++;
>  		}
>  		rb_desc = __next_ring_buffer_desc(rb_desc);
>  	}
> 
> base-commit: 8cdeaa50eae8dad34885515f62559ee83e7e8dda
> prerequisite-patch-id: 8bdd7b28f09e97cff4f5995196b83b377c961d8b
> prerequisite-patch-id: 6781b7ce5156d229fd21a757ef0cd66dd76130cc
> prerequisite-patch-id: 56bcbbdceddf4533770d48d1835d5b7e11f47d56
> -- 
> 2.39.5
>