[REGRESSION]: [PATCH mm-hotfixes v5 3/5] x86/mm/pat: acquire init_mm read lock on attribute change to avoid UAF

["Borah, Chaitanya Kumar" <chaitanya.kumar.borah@intel.com>]

Hello Lorenzo,

On 7/17/2026 11:00 PM, Lorenzo Stoakes (ARM) wrote:
> A previous commit protected us against races between ptdump and CPA
> collapse, however one still exists between attribute changes and collapse
> as reported by Denis V. Lunev (linked).
> 
> When an attribute change arises, a lockless page table walker obtains a PTE
> entry, which is later written to via set_pte_atomic():
> 
> ...
> -> change_page_attr_set_clr()
> -> __change_page_attr_set_clr()
> -> __change_page_attr()
> 	-> _lookup_address_cpa()
> 	-> lookup_address_in_pgd_attr()
> 	-> [ lockless page table walker ]
> -> set_pte_atomic()
> 
> There is nothing preventing a concurrent CPA collapse which can free the
> PTE that was retrieved here, resulting in a use-after-free.
> 
> With the mmap write lock taken on init_mm over CPA collapse, we can now
> resolve this race by acquiring an mmap read lock on init_mm over
> __change_page_attr_set_clr().
> 
> This locks across the whole operation over which the walk and the PTE entry
> write occurs, solving the race.
> 
> It is safe to do this here, as no spinlocks are held upon entry to
> __change_page_attr_set_clr().
> 
> The CPA_COLLAPSE flag is only set by set_memory_rox(), which exclusively
> operates upon vmalloc ranges, and on x86 only within the module mapping
> space.
> 
> This is important, because some callers directly invoke
> __change_page_attr_set_clr(), bypassing this lock. However, none of these
> operate within the module mapping space.
> 
> * cpa_process_alias() - a recursive helper called by
>    __change_page_attr_set_clr().
> * __set_memory_enc_pgtable() - operates on the direct mapping and (via
>    __vmbus_establish_gpadl()) the vmalloc mapping space.
> * __set_pages_[n]p() - called by set_direct_map_[invalid, default,
>    valid]_noflush(), __kernel_map_pages() - operates on the direct map.
> * kernel_[un]map_pages_in_pgd() - operates on EFI ranges.
> 
> This work is based upon Denis V. Lunev's excellent analysis of the bug with
> gratitude.
> 

This seems to be causing regression in our linux-next CI [1] on some of 
the older machines (fi-elk-e7500, fi-ilk-650, fi-pnv-d510)

<4> [196.236867] ======================================================
<4> [196.236878] WARNING: possible circular locking dependency detected
<4> [196.236890] 7.2.0-rc4-next-20260720-next-20260720-g3fe08b9796f3+ #1 
Tainted: G S   U       L
<4> [196.236906] ------------------------------------------------------
<4> [196.236916] core_hotunplug/5687 is trying to acquire lock:
<4> [196.236927] ffffffff839048b8 ((init_mm).mmap_lock){++++}-{4:4}, at: 
change_page_attr_set_clr+0x10a/0x220
<4> [196.236958]
but task is already holding lock:
<4> [196.236969] ffff88810c852238 (&vm->mutex){+.+.}-{4:4}, at: 
i915_ggtt_driver_release+0xab/0x260 [i915]
<4> [196.237572]
which lock already depends on the new lock.
<4> [196.237585]
the existing dependency chain (in reverse order) is:
<4> [196.237597]
-> #2 (&vm->mutex){+.+.}-{4:4}:
<4> [196.237617]        i915_gem_shrinker_taints_mutex+0x35/0x70 [i915]
<4> [196.238267]        i915_address_space_init+0x226/0x2a0 [i915]
<4> [196.238961]        i915_ggtt_init_hw+0x2c/0x140 [i915]
<4> [196.239638]        i915_driver_hw_probe+0x208/0x380 [i915]
<4> [196.240260]        i915_driver_probe+0x113/0x5b0 [i915]
<4> [196.240883]        i915_pci_probe+0xe0/0x1d0 [i915]

Detailed log can be seen found in [2]

We confirmed that reverting the patch solves the issue.

Could you please check why the patch causes this regression and provide
a fix if necessary?

Regards
Chaitanya

[1] https://intel-gfx-ci.01.org/tree/linux-next/combined-alt.html?
[2] 
https://intel-gfx-ci.01.org/tree/linux-next/next-20260720/fi-ilk-650/igt@core_hotunplug@unbind-rebind.html

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init_mm read lock on attribute change to avoid UAF
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init_mm write lock on collapse to avoid UAF
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# first bad commit: [b55b09b68a9cd81d2c1f4004e4c258cbfa61b16f] 
x86/mm/pat: acquire init_mm read lock on attribute change to avoid UAF


> Link: https://lore.kernel.org/all/20260626163213.2284080-1-den@openvz.org/
> Fixes: 41d88484c71c ("x86/mm/pat: restore large ROX pages after fragmentation")
> Cc: stable@vger.kernel.org
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
> ---
>   arch/x86/mm/pat/set_memory.c | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
> index d1e63f7d267f..301fb9e77d91 100644
> --- a/arch/x86/mm/pat/set_memory.c
> +++ b/arch/x86/mm/pat/set_memory.c
> @@ -2122,7 +2122,9 @@ static int change_page_attr_set_clr(unsigned long *addr, int numpages,
>   	cpa.curpage = 0;
>   	cpa.force_split = force_split;
>   
> -	ret = __change_page_attr_set_clr(&cpa, 1);
> +	/* Avoid race with concurrent CPA collapse. */
> +	scoped_guard(mmap_read_lock, &init_mm)
> +		ret = __change_page_attr_set_clr(&cpa, 1);
>   
>   	/*
>   	 * Check whether we really changed something:
>