[PATCH] mm/madvise: disallow guard region installation in uffd-armed VMAs

[PATCH] mm/madvise: disallow guard region installation in uffd-armed VMAs

[Lorenzo Stoakes (ARM)]

In order to establish guard regions, guard markers are installed in the
page tables of a mapped VMA.

However, problems arise when one tries to combine this with registering a
VMA for userfaultfd, as such operations may themselves need to install page
table markers such as uffd-wp.

When this installation is attempted the guard marker prevents these markers
being applied, which can lead to unexpected results (especially if the
guard regions are later removed.)

This is especially problematic because guard region installation causes the
regions to be zapped first, meaning that the removal of userfaultfd-tracked
ranges will not be logged in this case.

Therefore, disallow this.

This makes sense as conceptually, guard regions mark a part of a VMA as
effectively unmapped at the page table level, and removing the markers is
like mapping the VMA again, over which operation uffd markers would in any
case be lost.

Only check for this on installation, as a VMA can be registered for
userfaultfd after guard regions are established.

In this case, the guard regions cannot have new markers installed, and no
faults can occur in the guard regions, until they are removed, at which
point everything should function normally.

A survey of open source users indicates nobody is relying on installing
guard regions in uffd-armed VMAs. Software including glibc, android,
tcmalloc and CRIU were checked.

Fix the selftest to reflect the change.

Fixes: 662df3e5c376 ("mm: madvise: implement lightweight guard page mechanism")
Reported-by: Tal Zussman <tz2294@columbia.edu>
Closes: https://lore.kernel.org/all/20261111-uffd-state-fixes-v1-9-00713d368976@columbia.edu/
Closes: https://lore.kernel.org/all/20261111-uffd-state-fixes-v1-10-00713d368976@columbia.edu/
Closes: https://lore.kernel.org/all/20261111-uffd-state-fixes-v1-11-00713d368976@columbia.edu/
Cc: stable@vger.kernel.org
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---
 mm/madvise.c                               |  4 ++++
 tools/testing/selftests/mm/guard-regions.c | 12 ++++++++----
 2 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/mm/madvise.c b/mm/madvise.c
index 0214efd2a014..268731e90f12 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -1156,6 +1156,10 @@ static long madvise_guard_install(struct madvise_behavior *madv_behavior)
 	if (!is_valid_guard_vma(vma, /* allow_locked = */false))
 		return -EINVAL;
 
+	/* Guard markers are incompatible with UFFD markers. */
+	if (userfaultfd_armed(vma))
+		return -EINVAL;
+
 	/*
 	 * Set atomically under read lock. All pertinent readers will need to
 	 * acquire an mmap/VMA write lock to read it. All remaining readers may
diff --git a/tools/testing/selftests/mm/guard-regions.c b/tools/testing/selftests/mm/guard-regions.c
index 02f69244f9a4..97da54dcf3a4 100644
--- a/tools/testing/selftests/mm/guard-regions.c
+++ b/tools/testing/selftests/mm/guard-regions.c
@@ -1476,7 +1476,7 @@ TEST_F(guard_regions, cold_pageout)
 	ASSERT_EQ(munmap(ptr, 10 * page_size), 0);
 }
 
-/* Ensure that guard pages do not break userfaultd. */
+/* Ensure that guard regions cannot be installed in uffd-armed VMAs. */
 TEST_F(guard_regions, uffd)
 {
 	const unsigned long page_size = self->page_size;
@@ -1526,10 +1526,15 @@ TEST_F(guard_regions, uffd)
 	reg.mode = UFFDIO_REGISTER_MODE_MISSING;
 	ASSERT_EQ(ioctl(uffd, UFFDIO_REGISTER, &reg), 0);
 
-	/* Guard the range. This should not trigger the uffd. */
+	/* Now try to guard, should fail with EINVAL. */
+	ASSERT_EQ(madvise(ptr, 10 * page_size, MADV_GUARD_INSTALL), -1);
+	ASSERT_EQ(errno, EINVAL);
+
+	/* Once unregistered, we should be able to guard the range. */
+	ASSERT_EQ(ioctl(uffd, UFFDIO_UNREGISTER, &range), 0);
 	ASSERT_EQ(madvise(ptr, 10 * page_size, MADV_GUARD_INSTALL), 0);
 
-	/* The guarding should behave as usual with no uffd intervention. */
+	/* The guarding should behave as usual. */
 	for (i = 0; i < 10; i++) {
 		char *curr = &ptr[i * page_size];
 
@@ -1537,7 +1542,6 @@ TEST_F(guard_regions, uffd)
 	}
 
 	/* Cleanup. */
-	ASSERT_EQ(ioctl(uffd, UFFDIO_UNREGISTER, &range), 0);
 	close(uffd);
 	ASSERT_EQ(munmap(ptr, 10 * page_size), 0);
 }

---
base-commit: 4318887a3efa4c53d6f083ccfd7105b809a3ac65
change-id: 20261112-disallow-uffd-guard-install-a5294129ad7b

Cheers,
-- 
Lorenzo Stoakes (ARM) <ljs@kernel.org>

Re: [PATCH] mm/madvise: disallow guard region installation in uffd-armed VMAs

[Liam R. Howlett]

On 26/11/12 05:38PM, Lorenzo Stoakes (ARM) wrote:
> In order to establish guard regions, guard markers are installed in the
> page tables of a mapped VMA.
> 
> However, problems arise when one tries to combine this with registering a
> VMA for userfaultfd, as such operations may themselves need to install page
> table markers such as uffd-wp.
> 
> When this installation is attempted the guard marker prevents these markers
> being applied, which can lead to unexpected results (especially if the
> guard regions are later removed.)
> 
> This is especially problematic because guard region installation causes the
> regions to be zapped first, meaning that the removal of userfaultfd-tracked
> ranges will not be logged in this case.
> 
> Therefore, disallow this.
> 
> This makes sense as conceptually, guard regions mark a part of a VMA as
> effectively unmapped at the page table level, and removing the markers is
> like mapping the VMA again, over which operation uffd markers would in any
> case be lost.
> 
> Only check for this on installation, as a VMA can be registered for
> userfaultfd after guard regions are established.
> 
> In this case, the guard regions cannot have new markers installed, and no
> faults can occur in the guard regions, until they are removed, at which
> point everything should function normally.
> 
> A survey of open source users indicates nobody is relying on installing
> guard regions in uffd-armed VMAs. Software including glibc, android,
> tcmalloc and CRIU were checked.
> 
> Fix the selftest to reflect the change.
> 
> Fixes: 662df3e5c376 ("mm: madvise: implement lightweight guard page mechanism")
> Reported-by: Tal Zussman <tz2294@columbia.edu>
> Closes: https://lore.kernel.org/all/20261111-uffd-state-fixes-v1-9-00713d368976@columbia.edu/
> Closes: https://lore.kernel.org/all/20261111-uffd-state-fixes-v1-10-00713d368976@columbia.edu/
> Closes: https://lore.kernel.org/all/20261111-uffd-state-fixes-v1-11-00713d368976@columbia.edu/
> Cc: stable@vger.kernel.org
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

Reviewed-by: Liam R. Howlett (Google) <liam@infradead.org>

> ---
>  mm/madvise.c                               |  4 ++++
>  tools/testing/selftests/mm/guard-regions.c | 12 ++++++++----
>  2 files changed, 12 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 0214efd2a014..268731e90f12 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -1156,6 +1156,10 @@ static long madvise_guard_install(struct madvise_behavior *madv_behavior)
>  	if (!is_valid_guard_vma(vma, /* allow_locked = */false))
>  		return -EINVAL;
>  
> +	/* Guard markers are incompatible with UFFD markers. */
> +	if (userfaultfd_armed(vma))
> +		return -EINVAL;
> +
>  	/*
>  	 * Set atomically under read lock. All pertinent readers will need to
>  	 * acquire an mmap/VMA write lock to read it. All remaining readers may
> diff --git a/tools/testing/selftests/mm/guard-regions.c b/tools/testing/selftests/mm/guard-regions.c
> index 02f69244f9a4..97da54dcf3a4 100644
> --- a/tools/testing/selftests/mm/guard-regions.c
> +++ b/tools/testing/selftests/mm/guard-regions.c
> @@ -1476,7 +1476,7 @@ TEST_F(guard_regions, cold_pageout)
>  	ASSERT_EQ(munmap(ptr, 10 * page_size), 0);
>  }
>  
> -/* Ensure that guard pages do not break userfaultd. */
> +/* Ensure that guard regions cannot be installed in uffd-armed VMAs. */
>  TEST_F(guard_regions, uffd)
>  {
>  	const unsigned long page_size = self->page_size;
> @@ -1526,10 +1526,15 @@ TEST_F(guard_regions, uffd)
>  	reg.mode = UFFDIO_REGISTER_MODE_MISSING;
>  	ASSERT_EQ(ioctl(uffd, UFFDIO_REGISTER, &reg), 0);
>  
> -	/* Guard the range. This should not trigger the uffd. */
> +	/* Now try to guard, should fail with EINVAL. */
> +	ASSERT_EQ(madvise(ptr, 10 * page_size, MADV_GUARD_INSTALL), -1);
> +	ASSERT_EQ(errno, EINVAL);
> +
> +	/* Once unregistered, we should be able to guard the range. */
> +	ASSERT_EQ(ioctl(uffd, UFFDIO_UNREGISTER, &range), 0);
>  	ASSERT_EQ(madvise(ptr, 10 * page_size, MADV_GUARD_INSTALL), 0);
>  
> -	/* The guarding should behave as usual with no uffd intervention. */
> +	/* The guarding should behave as usual. */
>  	for (i = 0; i < 10; i++) {
>  		char *curr = &ptr[i * page_size];
>  
> @@ -1537,7 +1542,6 @@ TEST_F(guard_regions, uffd)
>  	}
>  
>  	/* Cleanup. */
> -	ASSERT_EQ(ioctl(uffd, UFFDIO_UNREGISTER, &range), 0);
>  	close(uffd);
>  	ASSERT_EQ(munmap(ptr, 10 * page_size), 0);
>  }
> 
> ---
> base-commit: 4318887a3efa4c53d6f083ccfd7105b809a3ac65
> change-id: 20261112-disallow-uffd-guard-install-a5294129ad7b
> 
> Cheers,
> -- 
> Lorenzo Stoakes (ARM) <ljs@kernel.org>
> 

Re: [PATCH] mm/madvise: disallow guard region installation in uffd-armed VMAs

[David Hildenbrand (Arm)]

On 11/12/26 18:38, Lorenzo Stoakes (ARM) wrote:
> In order to establish guard regions, guard markers are installed in the
> page tables of a mapped VMA.
> 
> However, problems arise when one tries to combine this with registering a
> VMA for userfaultfd, as such operations may themselves need to install page
> table markers such as uffd-wp.

How does this interact with UFFDIO_POISON markers that can outlive even
userfaultfd registration?

I'd assume it's just the same thing, just like having proper hwpoison marker in
anon mappings, so it just works (uffd-wp will skip, guard installation will zap).

> 
> When this installation is attempted the guard marker prevents these markers
> being applied, which can lead to unexpected results (especially if the
> guard regions are later removed.)
> 
> This is especially problematic because guard region installation causes the
> regions to be zapped first, meaning that the removal of userfaultfd-tracked
> ranges will not be logged in this case.

Again, similar to trying to uffd-wp-protect'ing a hwpoison marker (which will be
ignored), and then zapping that.

Isn't guard marker installation/removal just another form of zapping?

So to me this all sounds like "if user space does something stupid, it's user
space's fault".

> 
> Therefore, disallow this.
> 
> This makes sense as conceptually, guard regions mark a part of a VMA as
> effectively unmapped at the page table level, and removing the markers is
> like mapping the VMA again, over which operation uffd markers would in any
> case be lost.

Right, but that to me just says "on the parts of the VMA where you mess with
guard markers, your uffd-wp protection is similarly ignored".

So disallowing it entirely for a VMA, I am not sure. I could see use cases for
that, although you might argue that user space should then register uffd-wp on
the separate pieces. But then you get multiple VMAs again.


-- 
Cheers,

David

Re: [PATCH] mm/madvise: disallow guard region installation in uffd-armed VMAs

[Vlastimil Babka (SUSE)]

On 11/12/26 18:38, Lorenzo Stoakes (ARM) wrote:
> In order to establish guard regions, guard markers are installed in the
> page tables of a mapped VMA.
> 
> However, problems arise when one tries to combine this with registering a
> VMA for userfaultfd, as such operations may themselves need to install page
> table markers such as uffd-wp.
> 
> When this installation is attempted the guard marker prevents these markers
> being applied, which can lead to unexpected results (especially if the
> guard regions are later removed.)
> 
> This is especially problematic because guard region installation causes the
> regions to be zapped first, meaning that the removal of userfaultfd-tracked
> ranges will not be logged in this case.
> 
> Therefore, disallow this.
> 
> This makes sense as conceptually, guard regions mark a part of a VMA as
> effectively unmapped at the page table level, and removing the markers is
> like mapping the VMA again, over which operation uffd markers would in any
> case be lost.
> 
> Only check for this on installation, as a VMA can be registered for
> userfaultfd after guard regions are established.
> 
> In this case, the guard regions cannot have new markers installed, and no
> faults can occur in the guard regions, until they are removed, at which
> point everything should function normally.

So IIUC this means the other order of operations (first guard regions, then
uffd) already prevents the two co-existing. Makes sense to prevent the same
in the order of uffd and then guard regions then.

> A survey of open source users indicates nobody is relying on installing
> guard regions in uffd-armed VMAs. Software including glibc, android,
> tcmalloc and CRIU were checked.
> 
> Fix the selftest to reflect the change.
> 
> Fixes: 662df3e5c376 ("mm: madvise: implement lightweight guard page mechanism")
> Reported-by: Tal Zussman <tz2294@columbia.edu>
> Closes: https://lore.kernel.org/all/20261111-uffd-state-fixes-v1-9-00713d368976@columbia.edu/
> Closes: https://lore.kernel.org/all/20261111-uffd-state-fixes-v1-10-00713d368976@columbia.edu/
> Closes: https://lore.kernel.org/all/20261111-uffd-state-fixes-v1-11-00713d368976@columbia.edu/
> Cc: stable@vger.kernel.org
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>

> ---
>  mm/madvise.c                               |  4 ++++
>  tools/testing/selftests/mm/guard-regions.c | 12 ++++++++----
>  2 files changed, 12 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 0214efd2a014..268731e90f12 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -1156,6 +1156,10 @@ static long madvise_guard_install(struct madvise_behavior *madv_behavior)
>  	if (!is_valid_guard_vma(vma, /* allow_locked = */false))
>  		return -EINVAL;
>  
> +	/* Guard markers are incompatible with UFFD markers. */
> +	if (userfaultfd_armed(vma))
> +		return -EINVAL;
> +
>  	/*
>  	 * Set atomically under read lock. All pertinent readers will need to
>  	 * acquire an mmap/VMA write lock to read it. All remaining readers may
> diff --git a/tools/testing/selftests/mm/guard-regions.c b/tools/testing/selftests/mm/guard-regions.c
> index 02f69244f9a4..97da54dcf3a4 100644
> --- a/tools/testing/selftests/mm/guard-regions.c
> +++ b/tools/testing/selftests/mm/guard-regions.c
> @@ -1476,7 +1476,7 @@ TEST_F(guard_regions, cold_pageout)
>  	ASSERT_EQ(munmap(ptr, 10 * page_size), 0);
>  }
>  
> -/* Ensure that guard pages do not break userfaultd. */
> +/* Ensure that guard regions cannot be installed in uffd-armed VMAs. */
>  TEST_F(guard_regions, uffd)
>  {
>  	const unsigned long page_size = self->page_size;
> @@ -1526,10 +1526,15 @@ TEST_F(guard_regions, uffd)
>  	reg.mode = UFFDIO_REGISTER_MODE_MISSING;
>  	ASSERT_EQ(ioctl(uffd, UFFDIO_REGISTER, &reg), 0);
>  
> -	/* Guard the range. This should not trigger the uffd. */
> +	/* Now try to guard, should fail with EINVAL. */
> +	ASSERT_EQ(madvise(ptr, 10 * page_size, MADV_GUARD_INSTALL), -1);
> +	ASSERT_EQ(errno, EINVAL);
> +
> +	/* Once unregistered, we should be able to guard the range. */
> +	ASSERT_EQ(ioctl(uffd, UFFDIO_UNREGISTER, &range), 0);
>  	ASSERT_EQ(madvise(ptr, 10 * page_size, MADV_GUARD_INSTALL), 0);
>  
> -	/* The guarding should behave as usual with no uffd intervention. */
> +	/* The guarding should behave as usual. */
>  	for (i = 0; i < 10; i++) {
>  		char *curr = &ptr[i * page_size];
>  
> @@ -1537,7 +1542,6 @@ TEST_F(guard_regions, uffd)
>  	}
>  
>  	/* Cleanup. */
> -	ASSERT_EQ(ioctl(uffd, UFFDIO_UNREGISTER, &range), 0);
>  	close(uffd);
>  	ASSERT_EQ(munmap(ptr, 10 * page_size), 0);
>  }
> 
> ---
> base-commit: 4318887a3efa4c53d6f083ccfd7105b809a3ac65
> change-id: 20261112-disallow-uffd-guard-install-a5294129ad7b
> 
> Cheers,