[PATCH mm-hotfixes v2 2/4] x86/mm/pat: acquire mmap lock on page table free to avoid ptdump UAF

[Lorenzo Stoakes <ljs@kernel.org>]

x86 implements page attribute modification using its Change Page
Attributes (CPA) mechanism.

This tracks properties of ranges such as cache mode through x86 page
attributes, and as part of that logic manipulates kernel page tables.

Since commit 41d88484c71c ("x86/mm/pat: restore large ROX pages after
fragmentation") ranges of kernel page table entries can be collapsed into
huge page table entries as part of this logic.

As part of this collapse, it frees the page tables which the collapsed
entries previously pointed to, and it does so without any relevant locks
being held to preclude concurrent kernel page table walkers.

The only way this code can be reached is if CPA_COLLAPSE is specified, and
this is only set in set_memory_rox() via:

set_memory_rox()
-> change_page_attr_set_clr()
-> cpa_flush()
-> cpa_collapse_large_pages()

Notable users of this are execmem and bpf when manipulating executable
mappings.

However, this is problematic for ptdump, as it walks ranges it does not own
and thus runs the risk of a use-after-free on page tables freed underneath
it.

This patch resolves the issue by acquiring the mmap read lock on init_mm to
provide mutual exclusion against ptdump, which acquires the init_mm write
lock.

It is safe to acquire a sleeping lock as all the callers invoke
set_memory_rox() from process context and in any case,
change_page_attr_set_clr() calls vm_unmap_alias() which ultimately takes a
mutex, disallowing atomic context here.

We also include cleanup.h in order to use a scoped_guard() to implement
this cleanly.

Fixes: 41d88484c71c ("x86/mm/pat: restore large ROX pages after fragmentation")
Cc: stable@vger.kernel.org
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Signed-off-by: Lorenzo Stoakes <ljs@kernel.org>
---
 arch/x86/mm/pat/set_memory.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
index d023a40a1e03..4c4b8244502f 100644
--- a/arch/x86/mm/pat/set_memory.c
+++ b/arch/x86/mm/pat/set_memory.c
@@ -22,6 +22,7 @@
 #include <linux/cc_platform.h>
 #include <linux/set_memory.h>
 #include <linux/memregion.h>
+#include <linux/cleanup.h>
 
 #include <asm/e820/api.h>
 #include <asm/processor.h>
@@ -436,9 +437,16 @@ static void cpa_collapse_large_pages(struct cpa_data *cpa)
 
 	flush_tlb_all();
 
-	list_for_each_entry_safe(ptdesc, tmp, &pgtables, pt_list) {
-		list_del(&ptdesc->pt_list);
-		pagetable_free(ptdesc);
+	/*
+	 * ptdump might read these page tables, so avoid a use-after-free by
+	 * acquiring the mmap read lock on init_mm (ptdump acquires the mmap
+	 * write lock).
+	 */
+	scoped_guard(mmap_read_lock, &init_mm) {
+		list_for_each_entry_safe(ptdesc, tmp, &pgtables, pt_list) {
+			list_del(&ptdesc->pt_list);
+			pagetable_free(ptdesc);
+		}
 	}
 }
 

-- 
2.55.0