FAILED: patch "[PATCH] x86/shstk: Prevent deadlock during shstk sigreturn" failed to apply to 6.12-stable tree

The patch below does not apply to the 6.12-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.12.y
git checkout FETCH_HEAD
git cherry-pick -x 9874b2917b9fbc30956fee209d3c4aa47201c64e
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026050436-breeches-reformat-d041@gregkh' --subject-prefix 'PATCH 6.12.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 9874b2917b9fbc30956fee209d3c4aa47201c64e Mon Sep 17 00:00:00 2001
From: Rick Edgecombe <[email protected]>
Date: Thu, 9 Apr 2026 11:43:30 -0700
Subject: [PATCH] x86/shstk: Prevent deadlock during shstk sigreturn

During sigreturn the shadow stack signal frame is popped. The kernel does this by reading the shadow stack using normal read accesses. When it can't assume the memory is shadow stack, it takes extra steps to makes sure it is reading actual shadow stack memory and not other normal readable memory. It does this by holding the mmap read lock while doing the access and checking the flags of the VMA.

Unfortunately that is not safe. If the read of the shadow stack sigframe hits a page fault, the fault handler will try to recursively grab another mmap read lock. This normally works ok, but if a writer on another CPU is also waiting, the second read lock could fail and cause a deadlock.

Fix this by not holding mmap lock during the read access to userspace.

Instead use mmap_lock_speculate_...() to watch for changes between dropping mmap lock and the userspace access. Retry if anything grabbed an mmap write lock in between and could have changed the VMA.

These mmap_lock_speculate_...() helpers use mm::mm_lock_seq, which is only available when PER_VMA_LOCK is configured. So make X86_USER_SHADOW_STACK depend on it. On x86, PER_VMA_LOCK is a default configuration for SMP kernels. So drop support for the other configs under the assumption that the !SMP shadow stack user base does not exist.

Currently there is a check that skips the lookup work when the SSP can be assumed to be on a shadow stack. While reorganizing the function, remove the optimization to make the tricky code flows more common, such that issues like this cannot escape detection for so long.

index 99bb5217649a..f3f7cb01d69d 100644
@@ -1885,6 +1885,7 @@ config X86_USER_SHADOW_STACK
 	bool "X86 userspace shadow stack"
 	depends on AS_WRUSS
 	depends on X86_64
+	depends on PER_VMA_LOCK
 	select ARCH_USES_HIGH_VMA_FLAGS
 	select ARCH_HAS_USER_SHADOW_STACK
 	select X86_CET

[ Upstream commit 9874b2917b9fbc30956fee209d3c4aa47201c64e ]

During sigreturn the shadow stack signal frame is popped. The kernel does this by reading the shadow stack using normal read accesses. When it can't assume the memory is shadow stack, it takes extra steps to makes sure it is reading actual shadow stack memory and not other normal readable memory. It does this by holding the mmap read lock while doing the access and checking the flags of the VMA.

Unfortunately that is not safe. If the read of the shadow stack sigframe hits a page fault, the fault handler will try to recursively grab another mmap read lock. This normally works ok, but if a writer on another CPU is also waiting, the second read lock could fail and cause a deadlock.

Fix this by doing the read of the userspace memory via gup. Embed it in the get_shstk_data() helper.

Currently there is a check that skips the lookup work when the SSP can be assumed to be on a shadow stack. While reorganizing the function, remove the optimization to make the tricky code flows more common, such that issues like this cannot escape detection for so long.

[Due to missing per-vma MM sequence counter, use a simpler GUP based solution for the backport]

---
 arch/x86/kernel/shstk.c | 46 ++++++++++++++++++++++++++---------------
 1 file changed, 29 insertions(+), 17 deletions(-)

index 0dc983b33b003..373a44a5c478f 100644
@@ -18,6 +18,7 @@
 #include <linux/sizes.h>
 #include <linux/user.h>
 #include <linux/syscalls.h>
+#include <linux/highmem.h>
 #include <asm/msr.h>
 #include <asm/fpu/xstate.h>
 #include <asm/fpu/types.h>

@@ -262,11 +263,29 @@ static int put_shstk_data(u64 __user *addr, u64 data)
 	return 0;
 }
 
+/* Copy from aligned address in userspace without risk of page fault. */
+static int shstk_copy_user_gup(unsigned long *ldata, unsigned long __user *addr)
+{
+	struct page *page;
+	void *kaddr;
+
+	mmap_assert_locked(current->mm);
+	if (get_user_pages((unsigned long)addr, 1, 0, &page) != 1)
+		return -EFAULT;
+
+	kaddr = kmap_local_page(page);
+	*ldata = *(unsigned long *)(kaddr + offset_in_page(addr));
+	kunmap_local(kaddr);
+	put_page(page);
+
+	return 0;
+}
+
 static int get_shstk_data(unsigned long *data, unsigned long __user *addr)
 {
 	unsigned long ldata;
 
-	if (unlikely(get_user(ldata, addr)))
+	if (shstk_copy_user_gup(&ldata, addr))
 		return -EFAULT;
 
 	if (!(ldata & SHSTK_DATA_BIT))

@@ -296,7 +315,6 @@ static int shstk_pop_sigframe(unsigned long *ssp)
 {
 	struct vm_area_struct *vma;
 	unsigned long token_addr;
-	bool need_to_check_vma;
 	int err = 1;
 
 	/*

@@ -308,26 +326,21 @@ static int shstk_pop_sigframe(unsigned long *ssp)
 	if (!IS_ALIGNED(*ssp, 8))
 		return -EINVAL;
 
-	need_to_check_vma = PAGE_ALIGN(*ssp) == *ssp;
-
-	if (need_to_check_vma)
-		if (mmap_read_lock_killable(current->mm))
-			return -EINTR;
+	if (mmap_read_lock_killable(current->mm))
+		return -EINTR;
 
 	err = get_shstk_data(&token_addr, (unsigned long __user *)*ssp);
 	if (unlikely(err))
 		goto out_err;
 
-	if (need_to_check_vma) {
-		vma = find_vma(current->mm, *ssp);
-		if (!vma || !(vma->vm_flags & VM_SHADOW_STACK)) {
-			err = -EFAULT;
-			goto out_err;
-		}
-
-		mmap_read_unlock(current->mm);
+	vma = find_vma(current->mm, *ssp);
+	if (!vma || !(vma->vm_flags & VM_SHADOW_STACK)) {
+		err = -EFAULT;
+		goto out_err;
 	}
 
+	mmap_read_unlock(current->mm);
+
 	/* Restore SSP aligned? */
 	if (unlikely(!IS_ALIGNED(token_addr, 8)))
 		return -EINVAL;

@@ -340,8 +353,7 @@ static int shstk_pop_sigframe(unsigned long *ssp)
 
 	return 0;
 out_err:
-	if (need_to_check_vma)
-		mmap_read_unlock(current->mm);
+	mmap_read_unlock(current->mm);
 	return err;
 }
 

2.54.0

Queued for 6.12 and 6.6, thanks.

--
Thanks,
Sasha