Re: [PATCH mm-hotfixes v5 0/5] mm: fix UAF caused by race between ptdump and vmap pgtable freeing

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 17 Jul 2026 18:30:06 +0100 "Lorenzo Stoakes (ARM)" <ljs@kernel.org> wrote:

> Kernel page table walkers fall into two broad categories - those ranges
> where no exclusion is required via walk_kernel_page_table_range_lockless()
> and those where exclusion is required via walk_kernel_page_table_range()
> or walk_page_range_debug().
> 
> The former category is used only by arm64 arch code operating on ranges it
> both wholly owns and does not concurrently write.
> 
> The latter category consists of kernel page table walkers operating on
> ranges that are wholly owned (but which need exclusion against concurrent
> writers).
> 
> The lock used for exclusion is the mmap lock, and for kernel ranges this
> the mmap lock on init_mm.
> 
> ptdump is a special case being both the only user of
> walk_page_range_debug(), and the only case in which it walks ranges it does
> not own.
> 
> This presents a problem, as page tables may be freed under ptdump. And
> indeed there is a use-after-free bug in the kernel as a result, which this
> series addresses.
> 
> ...
>
> This series works around this by #ifndef CONFIG_ARM64'ing the mmap read
> lock in vmap logic, then partially reverting commit fa93b45fd397 ("arm64:
> Enable vmalloc-huge with ptdump"), keeping the enablement of huge vmap
> support, and removing the ifdeffery with the partial revert patch.
> 

Thanks, I updated mm-hotfixes-unstable to this version.

> v5:
> * Rebased on latest master of Linus's tree.
> * Accumulated tags, thanks everybody!
> * Added additional commit to fix race between CPA collapse/attribute set.
> * Slight commit message tweaks.

Here's how v5 altered mm.git.  This is from the addition of [3/5] - no
other patches changed.


 arch/x86/mm/pat/set_memory.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/arch/x86/mm/pat/set_memory.c~b
+++ a/arch/x86/mm/pat/set_memory.c
@@ -2122,7 +2122,9 @@ static int change_page_attr_set_clr(unsi
 	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:
_