Re: [PATCH] bpf: devmap: Check attr->max_entries more carefully

[Mark Rutland]

On Mon, Oct 16, 2017 at 08:52:13PM +0200, Daniel Borkmann wrote:
> [ +Tejun, Mark, John ]
> 
> On 10/16/2017 12:00 AM, Richard Weinberger wrote:
> > max_entries is user controlled and used as input for __alloc_percpu().
> > This function expects that the allocation size is a power of two and
> > less than PCPU_MIN_UNIT_SIZE.
> > Otherwise a WARN() is triggered.
> > 
> > Fixes: 11393cc9b9be ("xdp: Add batching support to redirect map")
> > Reported-by: Shankara Pailoor <sp3485@xxxxxxxxxxxx>
> > Reported-by: syzkaller <syzkaller@xxxxxxxxxxxxxxxx>
> > Signed-off-by: Richard Weinberger <richard@xxxxxx>
> 
> Thanks for the patch, Richard. There was a prior discussion here [1] on
> the same issue, I thought this would have been resolved by now, but looks
> like it's still open and there was never a follow-up, at least I don't see
> it in the percpu tree if I didn't miss anything.

Sorry, this was on my todo list, but I've been bogged down with some
other work.

> I would suggest, we do the following below and pass __GFP_NOWARN from BPF
> side to the per-cpu allocs. This is kind of a generic 'issue' and we shouldn't
> add more code which bails out anyway just to work around the WARN(). Lets
> handle it properly instead.

Agreed. The below patch looks good to me, (with the suggested change to
the BPF side).

> If Tejun is fine with the one below, I could cook and official patch and
> cleanup the remaining call-sites from BPF which have similar pattern.

That would be great; thanks for taking this on.

Thanks,
Mark.

> 
>   [1] https://patchwork.kernel.org/patch/9975851/
> 
> Thanks,
> Daniel
> 
>  mm/percpu.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/percpu.c b/mm/percpu.c
> index 59d44d6..5d9414e 100644
> --- a/mm/percpu.c
> +++ b/mm/percpu.c
> @@ -1357,7 +1357,8 @@ static void __percpu *pcpu_alloc(size_t size, size_t align, bool reserved,
> 
>  	if (unlikely(!size || size > PCPU_MIN_UNIT_SIZE || align > PAGE_SIZE ||
>  		     !is_power_of_2(align))) {
> -		WARN(true, "illegal size (%zu) or align (%zu) for percpu allocation\n",
> +		WARN(!(gfp & __GFP_NOWARN),
> +		     "illegal size (%zu) or align (%zu) for percpu allocation\n",
>  		     size, align);
>  		return NULL;
>  	}
> @@ -1478,7 +1479,7 @@ static void __percpu *pcpu_alloc(size_t size, size_t align, bool reserved,
>  fail:
>  	trace_percpu_alloc_percpu_fail(reserved, is_atomic, size, align);
> 
> -	if (!is_atomic && warn_limit) {
> +	if (!is_atomic && warn_limit && !(gfp & __GFP_NOWARN)) {
>  		pr_warn("allocation failed, size=%zu align=%zu atomic=%d, %s\n",
>  			size, align, is_atomic, err);
>  		dump_stack();
> -- 
> 1.9.3