Re: [PATCH 0/2] mm: fix UAF caused by race between ptdump and vmap pgtable freeing

[Dev Jain <dev.jain@arm.com>]

On 12/07/26 2:16 pm, Lorenzo Stoakes wrote:
> On Sun, Jul 12, 2026 at 12:50:08PM +0530, Dev Jain wrote:
>> Will Deacon had pushed back on a similar approach:
>> https://lore.kernel.org/all/20250530123527.GA30463@willie-the-truck/
>>
>> Although now when I read back that thread, it feels more so like my
>> incompetency to convince :) because:
> 
> No haha not so, I think more like this stuff is fiddly.
> 
>>
>> 1. I don't think this pmd_free_pte_page() path is a hot path at all
> 
> Right, and we don't actually alter that path anyway
> 
>>
>> 2. We are doing a try lock which is almost guaranteed to succeed,
>>    so it's not like we are losing out on block mappings
> 
> Also it's specifically only on when vmap tries to make a mapping huge, and
> this path is being inconsistent with a convention that already existed - if
> you manipulate kernel page table mappings that can interact with other page
> table walkers, you have to take the init_mm mmap lock.
> 
>>
>> 3. Any overhead from the try lock will get dominated by the pgtable
>>    page free/TLB flush
> 
> Yup.
> 
>>
>> I guess you did not take the RCU approach because that would put code
>> into the generic kernel pgtable freeing path.
> 
> Well a number of reasons:
> 
> * firstly yes it makes the code path always RCU only to suit a specific
>   debug user as you say :)
> 
> * Importantly - we risk genuine RCU stall issues, because the ptdump then
>   has to be RCU too over vast ranges.
> 
>   To work around that you have to shard the ptdump walk, make an assumption
>   all callbacks are RCU-safe, and that the sharding suffices to avoid these
>   stalls.
> 
>   It's a ton of complexity and assumptions to account for... vmalloc doing
>   the wrong thing.
> 
> * It is an established precedent that we mmap lock init_mm for kernel page
>   table walking as per mm/pagewalk.c. It'd require significant rework there
>   and would disallow any future walkers like this if we were to require
>   RCU.
> 
> * The mmap lock approach is simple, safe, and as you say is only actually
>   required in code paths that manipulate page tables and thus are already
>   not hotpaths.
> 
> * If there's future work to free vmalloc page tables upon vunmap()
>   (currently it does not), we have a stable, established basis for doing so
>   that again puts the weight of the work on the operation being performed
>   rather than anything else.
> 
>>
>> I liked the RCU approach because I hate the fact that ptdump takes
>> an mmap_write_lock when it is literally only reading the pgtables.
> 
> Well you have to do that for the userland side, because there could be a
> concurrent downgraded mmap read lock during an munmap, and the same goes
> for non-VMA kernel ranges too, so it would have to keep doing that
> regardless.

Oh right, I didn't know x86 was using ptdump for user tables too.


> 
>> But your approach is simpler and fixes the problem at the particular spot
>> and not hammers the fix into a generic path. So overall, ACK.
> 
> Thanks!
> 
>>
>>
>>> Lorenzo Stoakes (2):
>>>       mm/vmalloc: acquire init_mm read lock on huge vmap promotion
>>>       Revert "arm64: Enable vmalloc-huge with ptdump"
>>>
>>>  arch/arm64/include/asm/ptdump.h |  2 --
>>>  arch/arm64/mm/mmu.c             | 43 ++++-------------------------------------
>>>  arch/arm64/mm/ptdump.c          | 11 ++---------
>>>  include/linux/mmap_lock.h       |  1 +
>>>  mm/pagewalk.c                   | 22 +++++++++++----------
>>>  mm/vmalloc.c                    | 41 ++++++++++++++++++++++++++++++---------
>>>  6 files changed, 51 insertions(+), 69 deletions(-)
>>> ---
>>> base-commit: a635d6748234582ea287c5ffeae28b9b23f91c7e
>>> change-id: 20260710-series-vmap-race-fix-2a4cac988938
>>>
>>> Cheers,
>>
> 
> Cheers, Lorenzo