On 1/31/22 17:11, Sean Christopherson wrote:
On Tue, Jan 25, 2022, Sean Christopherson wrote:
On Mon, Jan 24, 2022, Tadeusz Struk wrote:
On 1/24/22 10:29, Sean Christopherson wrote:
On Mon, Jan 24, 2022, Paolo Bonzini wrote:
On 1/24/22 18:26, Tadeusz Struk wrote:
Syzbot reported an use-after-free bug in update_accessed_dirty_bits().
Fix this by checking if the memremap'ed pointer is still valid.
access_ok only checks that the pointer is in the userspace range. Is this
correct? And if so, what are the exact circumstances in which access_ok
returns a non-NULL but also non-userspace address?
I "objected" to this patch in its initial posting[*]. AFAICT adding access_ok()
is just masking a more egregious bug where interpretation of vm_pgoff as a PFN
base is flat out wrong except for select backing stores that use VM_PFNMAP. In
other words, the vm_pgoff hack works for the /dev/mem use case, but it is wrong
in general.
The issue here is not related to /dev/mem, but binder allocated memory, which is
yet another special mapping use case. In this case the condition
if (!vma || !(vma->vm_flags & VM_PFNMAP))
doesn't cover this special mappings. Adding the access_ok() was my something
that fixed the use-after-free issue for me, and since I didn't have anything
better I thought I will send an RFC to start some discussion.
After some more debugging I came up with the bellow.
Will that be more acceptable?
I'm pretty sure anything that keeps the vm_pgoff "logic" is a band-aid. But I'm 99%
sure we can simply do cmpxchg directly on the user address, we just need to get
support for that, which has happily been posted[*]. I'll give that a shot tomorrow,
I want to convert similar code in the emulator, it'd be very nice to purge all of
this crud.
[*] https://lore.kernel.org/all/20220120160822.852009966@xxxxxxxxxxxxx
Posted a series and belatedly realized my script didn't pick up Debugged-by: to Cc
you :-/ Let me know if you want me to forward any/all of the series to you.
https://lore.kernel.org/all/20220201010838.1494405-1-seanjc@xxxxxxxxxx
It's fine, I can follow up using the link you sent.
I will try it tomorrow and let you know.
--
Thanks,
Tadeusz
