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

[Lorenzo Stoakes (ARM)]

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.

Link: https://lore.kernel.org/all/20260626163213.2284080-1-den@xxxxxxxxxx/
Fixes: 41d88484c71c ("x86/mm/pat: restore large ROX pages after fragmentation")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
---
 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:

-- 
2.55.0