Re: [PATCH v2] mm: memdup_user*() should use same gfp flags

[Michal Hocko <mhocko@suse.com>]

On Fri 22-01-21 19:47:42, Tetsuo Handa wrote:
> On 2021/01/22 10:35, Andrew Morton wrote:
> > On Wed, 20 Jan 2021 19:34:36 +0900 Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> wrote:
> > 
> >> syzbot is reporting that memdup_user_nul() which receives user-controlled
> >> size (which can be up to (INT_MAX & PAGE_MASK)) via vfs_write() will hit
> >> order >= MAX_ORDER path [1].

That is nasty!

> >> Let's add __GFP_NOWARN to memdup_user_nul() as with commit 6c8fcc096be9d02f
> >> ("mm: don't let userspace spam allocations warnings"). Also use GFP_USER as
> >> with commit 6c2c97a24f096e32 ("memdup_user(): switch to GFP_USER").

No, this is papering over a more troubling underlying problem. Userspace
shouldn't be able to trigger an aribitrary higher order allocations.
Those users with a large size to copy should be really using kvmalloc
based (e.g vmemdup_user).

> > That commit failed to explain why a switch to GFP_USER was performed,
> > so that commit isn't a good substitute for an explanation of this
> > change.
> 
> For example, commit 2f77d107050abc14 ("Fix incorrect user space access locking
> in mincore()") silently converted GFP_KERNEL to GFP_USER.
> 
>   #define GFP_KERNEL	(__GFP_RECLAIM | __GFP_IO | __GFP_FS)
>   #define GFP_USER	(__GFP_RECLAIM | __GFP_IO | __GFP_FS | __GFP_HARDWALL)
> 
>  * %GFP_KERNEL is typical for kernel-internal allocations. The caller requires
>  * %ZONE_NORMAL or a lower zone for direct access but can direct reclaim.
> 
>  * %GFP_USER is for userspace allocations that also need to be directly
>  * accessibly by the kernel or hardware. It is typically used by hardware
>  * for buffers that are mapped to userspace (e.g. graphics) that hardware
>  * still must DMA to. cpuset limits are enforced for these allocations.
> 
>  * %__GFP_HARDWALL enforces the cpuset memory allocation policy.
> 
> > 
> > So...  please fully describe the reason for this change right here in
> > this patch's changelog.
> 
> I guess that GFP_USER is chosen by cautious developers when memory is
> allocated by userspace request. Is there a guideline for when to use GFP_USER ?

I do not think we have anything better than the above. GFP_USER is
indeed used for userspace controlable allocations. So they can be a
subject to a more strict cpu policy. memdup_user_nul looks like a good
fit for GFP_USER to me. memdup_user and other variant already does this.

-- 
Michal Hocko
SUSE Labs