x86/mm: Fix check/use ordering in switch_mm_irqs_off()

This is a stable-specific fix. There is no single upstream commit to
cherry-pick; the equivalent mainline fix is commit 83b0177
("x86/mm: Fix SMP ordering in switch_mm_irqs_off()"), which is in
v6.18 but does not apply to these trees because the surrounding code
was refactored.

The backports of commit fea4e31 ("x86/mm: Eliminate window where
TLB flushes may be inadvertently skipped") -- the fix for
CVE-2025-37964 -- to the 6.1, 6.6 and 6.12 trees ended up with two
code blocks in the wrong order relative to mainline. The fix depends
on setting cpu_tlbstate.loaded_mm to LOADED_MM_SWITCHING *before*
reading tlb_gen, so that a concurrent TLB shootdown either sees
LOADED_MM_SWITCHING and sends the IPI, or the switching CPU sees the
updated tlb_gen. In the stable trees the write of LOADED_MM_SWITCHING
was placed after the tlb_gen read, so the race window fea4e31 was
meant to close is still open and CVE-2025-37964 is unfixed there:
a process can be left running with stale TLB entries, which typically
manifests as rare, hard-to-bisect memory corruption or segfaults.

This has been confirmed by several independent parties:

  - reproduced on 6.1.y/6.6.y/6.12.y with the test program in [1],
    and confirmed fixed by this patch
  - Seth Forshee saw segfaults on 6.12.y within ~30 minutes of running
    a test workload; with this patch it ran 18 hours cleanly [2]
  - Greg Thelen reports 6.6.y- and 6.12.y-based test failures fixed by
    this patch [3]

Dave Hansen acked the patch [4] and has no objection to it going into
stable [5].

The patch below is against 6.12.y; the identical change applies to
6.1.y and 6.6.y. (The cpumask_test_cpu()/smp_mb() portion of the
mainline fix is not needed here because commit 209954c
("x86/mm/tlb: Update mm_cpumask lazily") was never backported to these
trees.)

[1] https://lore.kernel.org/lkml/CAHDw0oGd0B4=uuv8NGqbUQ_ZVmSheU2bN70e4QhFXWvuAZdt2w@mail.gmail.com/
[2] https://lore.kernel.org/lkml/aZYWXe739XUJrBld@do-x1carbon/
[3] https://lore.kernel.org/lkml/CAHH2K0brx9omC9QYyB6Lio3t_1Lf8v=VaFoiaG23UgQ-aec89Q@mail.gmail.com/
[4] https://lore.kernel.org/lkml/281e8018-5506-4a79-8775-e0de7e58b95f@intel.com/
[5] https://lore.kernel.org/lkml/86421ee5-5332-46c2-bb48-d40310b818be@intel.com/

Fixes: fea4e31 ("x86/mm: Eliminate window where TLB flushes may be inadvertently skipped")  # 6.1.y/6.6.y/6.12.y backports
Acked-by: Dave Hansen <dave.hansen@intel.com>
Tested-by: Seth Forshee <sforshee@kernel.org>
Tested-by: Eric Hagberg <ehagberg@janestreet.com>
Signed-off-by: Stephen Dolan <sdolan@janestreet.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: stedolan

arch/x86/mm/tlb.c

@@ -612,6 +612,14 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
 		 */
 		cond_mitigation(tsk);
 
+		/*
+		 * Indicate that CR3 is about to change. nmi_uaccess_okay()
+		 * and others are sensitive to the window where mm_cpumask(),
+		 * CR3 and cpu_tlbstate.loaded_mm are not all in sync.
+		 */
+		this_cpu_write(cpu_tlbstate.loaded_mm, LOADED_MM_SWITCHING);
+		barrier();
+
 		/*
 		 * Stop remote flushes for the previous mm.
 		 * Skip kernel threads; we never send init_mm TLB flushing IPIs,
@@ -629,14 +637,6 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
 		next_tlb_gen = atomic64_read(&next->context.tlb_gen);
 
 		choose_new_asid(next, next_tlb_gen, &new_asid, &need_flush);
-
-		/*
-		 * Indicate that CR3 is about to change. nmi_uaccess_okay()
-		 * and others are sensitive to the window where mm_cpumask(),
-		 * CR3 and cpu_tlbstate.loaded_mm are not all in sync.
- 		 */
-		this_cpu_write(cpu_tlbstate.loaded_mm, LOADED_MM_SWITCHING);
-		barrier();
 	}
 
 	new_lam = mm_lam_cr3_mask(next);