Re: [PATCH][next] wifi: brcmfmac: fweh: Fix boot crash on Raspberry Pi 4

[Kees Cook <keescook@chromium.org>]

On Fri, Feb 16, 2024 at 01:27:56PM -0600, Gustavo A. R. Silva wrote:
> Fix boot crash on Raspberry Pi by moving the update to `event->datalen`
> before data is copied into flexible-array member `data` via `memcpy()`.
> 
> Flexible-array member `data` was annotated with `__counted_by(datalen)`
> in commit 62d19b358088 ("wifi: brcmfmac: fweh: Add __counted_by for
> struct brcmf_fweh_queue_item and use struct_size()"). The intention of
> this is to gain visibility into the size of `data` at run-time through
> its _counter_ (in this case `datalen`), and with this have its accesses
> bounds-checked at run-time via CONFIG_FORTIFY_SOURCE and
> CONFIG_UBSAN_BOUNDS.
> 
> To effectively accomplish the above, we shall update the counter
> (`datalen`), before the first access to the flexible array (`data`),
> which was also done in the mentioned commit.
> 
> However, commit edec42821911 ("wifi: brcmfmac: allow per-vendor event
> handling") inadvertently caused a buffer overflow, detected by
> FORTIFY_SOURCE. It moved the `event->datalen = datalen;` update to after
> the first `data` access, at which point `event->datalen` was not yet
> updated from zero (after calling `kzalloc()`), leading to the overflow
> issue.
> 
> This fix repositions the `event->datalen = datalen;` update before
> accessing `data`, restoring the intended buffer overflow protection. :)
> 
> Fixes: edec42821911 ("wifi: brcmfmac: allow per-vendor event handling")
> Reported-by: Nathan Chancellor <nathan@kernel.org>
> Closes: https://gist.github.com/nathanchance/e22f681f3bfc467f15cdf6605021aaa6
> Tested-by: Nathan Chancellor <nathan@kernel.org>
> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

Yup, this looks correct. Thanks!

Reviewed-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook