[PATCH 6.1 0874/1191] ARM: 9485/1: mm: acquire mmap write lock around show_pte() for user faults

6.1-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Xie Yuanbin <redacted>

[ Upstream commit 1039bffd6ae9c75b42b7d148d6c1106134107b66 ]

When CONFIG_DEBUG_USER=y, and cmdline "user_debug=31" is set,
a user fault may trigger show_pte() without any lock.
If another thread in the same process concurrently calls munmap(),
the page table pages may be freed while show_pte() is still traversing
them, causing a use-after-free in show_pte().

If CONFIG_ARM_LPAE=y, this may cause a kernel panic if the pages table
of PMD are freed when show_pte() is running.

Acquire mmap_write_lock() around show_pte() for user faults to fix the
contention.

For user faults, additionally restrict that show_pte() is called only
when the addr is a user-space address (addr < TASK_SIZE). This is because
the lock of tsk->mm only protects the virtual memory of user address space,
furthermore, dumping the page tables of a kernel-space address for user
faults is unnecessary and may have security implications.

Keep everything unchanged for kernel faults, because the kernel is
already in the "oops" state, acquiring a lock may risk a deadlock.

Co-developed-by: Qi Xi <redacted>

Fixes: 6d021b724481 ("ARM: dump pgd, pmd and pte states on unhandled data abort faults")
Link: https://lore.kernel.org/20260716014022.2823-1-xieyuanbin1@huawei.com (local)
Acked-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Reviewed-by: Linus Walleij <linusw@kernel.org>
Signed-off-by: Qi Xi <redacted>
Signed-off-by: Xie Yuanbin <redacted>
Signed-off-by: Russell King <redacted>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 arch/arm/mm/fault.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/arch/arm/mm/fault.c b/arch/arm/mm/fault.c
index d0681285dbda3..29a995c6d7c6b 100644
--- a/arch/arm/mm/fault.c
+++ b/arch/arm/mm/fault.c

@@ -201,7 +201,11 @@ __do_user_fault(unsigned long addr, unsigned int fsr, unsigned int sig,
 		pr_err("8<--- cut here ---\n");
 		pr_err("%s: unhandled page fault (%d) at 0x%08lx, code 0x%03x\n",
 		       tsk->comm, sig, addr, fsr);
-		show_pte(KERN_ERR, tsk->mm, addr);
+		if (likely(addr < TASK_SIZE)) {
+			mmap_write_lock(tsk->mm);
+			show_pte(KERN_ERR, tsk->mm, addr);
+			mmap_write_unlock(tsk->mm);
+		}
 		show_regs(regs);
 	}
 #endif

@@ -582,7 +586,15 @@ do_DataAbort(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 	pr_alert("8<--- cut here ---\n");
 	pr_alert("Unhandled fault: %s (0x%03x) at 0x%08lx\n",
 		inf->name, fsr, addr);
-	show_pte(KERN_ALERT, current->mm, addr);
+	if (likely(user_mode(regs))) {
+		if (addr < TASK_SIZE) {
+			mmap_write_lock(current->mm);
+			show_pte(KERN_ALERT, current->mm, addr);
+			mmap_write_unlock(current->mm);
+		}
+	} else {
+		show_pte(KERN_ALERT, current->mm, addr);
+	}
 
 	arm_notify_die("", regs, inf->sig, inf->code, (void __user *)addr,
 		       fsr, 0);

-- 
2.53.0