[PATCH v4 0/2] userfaultfd: do not untag user pointers

If a user program uses userfaultfd on ranges of heap memory, it may
end up passing a tagged pointer to the kernel in the range.start
field of the UFFDIO_REGISTER ioctl. This can happen when using an
MTE-capable allocator, or on Android if using the Tagged Pointers
feature for MTE readiness [1].

When a fault subsequently occurs, the tag is stripped from the fault
address returned to the application in the fault.address field
of struct uffd_msg. However, from the application's perspective,
the tagged address *is* the memory address, so if the application
is unaware of memory tags, it may get confused by receiving an
address that is, from its point of view, outside of the bounds of the
allocation. We observed this behavior in the kselftest for userfaultfd
[2] but other applications could have the same problem.

Address this by not untagging pointers passed to the userfaultfd
ioctls. Instead, let the system call fail. Also change the kselftest
to use mmap so that it doesn't encounter this problem.

[1] https://source.android.com/devices/tech/debug/tagged-pointers
[2] tools/testing/selftests/vm/userfaultfd.c

Peter Collingbourne (2):
  userfaultfd: do not untag user pointers
  selftest: use mmap instead of posix_memalign to allocate memory

 Documentation/arm64/tagged-address-abi.rst | 26 +++++++++++++++-------
 fs/userfaultfd.c                           | 26 ++++++++++------------
 tools/testing/selftests/vm/userfaultfd.c   |  6 +++--
 3 files changed, 34 insertions(+), 24 deletions(-)

-- 
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If a user program uses userfaultfd on ranges of heap memory, it may
end up passing a tagged pointer to the kernel in the range.start
field of the UFFDIO_REGISTER ioctl. This can happen when using an
MTE-capable allocator, or on Android if using the Tagged Pointers
feature for MTE readiness [1].

When a fault subsequently occurs, the tag is stripped from the fault
address returned to the application in the fault.address field
of struct uffd_msg. However, from the application's perspective,
the tagged address *is* the memory address, so if the application
is unaware of memory tags, it may get confused by receiving an
address that is, from its point of view, outside of the bounds of the
allocation. We observed this behavior in the kselftest for userfaultfd
[2] but other applications could have the same problem.

Address this by not untagging pointers passed to the userfaultfd
ioctls. Instead, let the system call fail. This will provide an
early indication of problems with tag-unaware userspace code instead
of letting the code get confused later, and is consistent with how
we decided to handle brk/mmap/mremap in commit dcde237319e6 ("mm:
Avoid creating virtual address aliases in brk()/mmap()/mremap()"),
as well as being consistent with the existing tagged address ABI
documentation relating to how ioctl arguments are handled.

The code change is a revert of commit 7d0325749a6c ("userfaultfd:
untag user pointers") plus some fixups to some additional calls to
validate_range that have appeared since then.

[1] https://source.android.com/devices/tech/debug/tagged-pointers
[2] tools/testing/selftests/vm/userfaultfd.c

Signed-off-by: Peter Collingbourne <redacted>
Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
Link: https://linux-review.googlesource.com/id/I761aa9f0344454c482b83fcfcce547db0a25501b
Fixes: 63f0c6037965 ("arm64: Introduce prctl() options to control the tagged user addresses ABI")
Cc: <redacted> # 5.4
---
v4:
- document the changes more accurately
- fix new calls to validate_range

 Documentation/arm64/tagged-address-abi.rst | 26 +++++++++++++++-------
 fs/userfaultfd.c                           | 26 ++++++++++------------
 2 files changed, 30 insertions(+), 22 deletions(-)

diff --git a/Documentation/arm64/tagged-address-abi.rst b/Documentation/arm64/tagged-address-abi.rst
index 459e6b66ff68..0c9120ec58ae 100644
--- a/Documentation/arm64/tagged-address-abi.rst
+++ b/Documentation/arm64/tagged-address-abi.rst

@@ -45,14 +45,24 @@ how the user addresses are used by the kernel:
 
 1. User addresses not accessed by the kernel but used for address space
    management (e.g. ``mprotect()``, ``madvise()``). The use of valid
-   tagged pointers in this context is allowed with the exception of
-   ``brk()``, ``mmap()`` and the ``new_address`` argument to
-   ``mremap()`` as these have the potential to alias with existing
-   user addresses.
-
-   NOTE: This behaviour changed in v5.6 and so some earlier kernels may
-   incorrectly accept valid tagged pointers for the ``brk()``,
-   ``mmap()`` and ``mremap()`` system calls.
+   tagged pointers in this context is allowed with these exceptions:
+
+   - ``brk()``, ``mmap()`` and the ``new_address`` argument to
+     ``mremap()`` as these have the potential to alias with existing
+      user addresses.
+
+     NOTE: This behaviour changed in v5.6 and so some earlier kernels may
+     incorrectly accept valid tagged pointers for the ``brk()``,
+     ``mmap()`` and ``mremap()`` system calls.
+
+   - The ``range.start``, ``start`` and ``dst`` arguments to the
+     ``UFFDIO_*`` ``ioctl()``s used on a file descriptor obtained from
+     ``userfaultfd()``, as fault addresses subsequently obtained by reading
+     the file descriptor will be untagged, which may otherwise confuse
+     tag-unaware programs.
+
+     NOTE: This behaviour changed in v5.14 and so some earlier kernels may
+     incorrectly accept valid tagged pointers for this system call.
 
 2. User addresses accessed by the kernel (e.g. ``write()``). This ABI
    relaxation is disabled by default and the application thread needs to

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index dd7a6c62b56f..27af6b82a758 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c

@@ -1236,23 +1236,21 @@ static __always_inline void wake_userfault(struct userfaultfd_ctx *ctx,
 }
 
 static __always_inline int validate_range(struct mm_struct *mm,
-					  __u64 *start, __u64 len)
+					  __u64 start, __u64 len)
 {
 	__u64 task_size = mm->task_size;
 
-	*start = untagged_addr(*start);
-
-	if (*start & ~PAGE_MASK)
+	if (start & ~PAGE_MASK)
 		return -EINVAL;
 	if (len & ~PAGE_MASK)
 		return -EINVAL;
 	if (!len)
 		return -EINVAL;
-	if (*start < mmap_min_addr)
+	if (start < mmap_min_addr)
 		return -EINVAL;
-	if (*start >= task_size)
+	if (start >= task_size)
 		return -EINVAL;
-	if (len > task_size - *start)
+	if (len > task_size - start)
 		return -EINVAL;
 	return 0;
 }

@@ -1313,7 +1311,7 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
 		vm_flags |= VM_UFFD_MINOR;
 	}
 
-	ret = validate_range(mm, &uffdio_register.range.start,
+	ret = validate_range(mm, uffdio_register.range.start,
 			     uffdio_register.range.len);
 	if (ret)
 		goto out;

@@ -1519,7 +1517,7 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
 	if (copy_from_user(&uffdio_unregister, buf, sizeof(uffdio_unregister)))
 		goto out;
 
-	ret = validate_range(mm, &uffdio_unregister.start,
+	ret = validate_range(mm, uffdio_unregister.start,
 			     uffdio_unregister.len);
 	if (ret)
 		goto out;

@@ -1668,7 +1666,7 @@ static int userfaultfd_wake(struct userfaultfd_ctx *ctx,
 	if (copy_from_user(&uffdio_wake, buf, sizeof(uffdio_wake)))
 		goto out;
 
-	ret = validate_range(ctx->mm, &uffdio_wake.start, uffdio_wake.len);
+	ret = validate_range(ctx->mm, uffdio_wake.start, uffdio_wake.len);
 	if (ret)
 		goto out;
 

@@ -1708,7 +1706,7 @@ static int userfaultfd_copy(struct userfaultfd_ctx *ctx,
 			   sizeof(uffdio_copy)-sizeof(__s64)))
 		goto out;
 
-	ret = validate_range(ctx->mm, &uffdio_copy.dst, uffdio_copy.len);
+	ret = validate_range(ctx->mm, uffdio_copy.dst, uffdio_copy.len);
 	if (ret)
 		goto out;
 	/*

@@ -1765,7 +1763,7 @@ static int userfaultfd_zeropage(struct userfaultfd_ctx *ctx,
 			   sizeof(uffdio_zeropage)-sizeof(__s64)))
 		goto out;
 
-	ret = validate_range(ctx->mm, &uffdio_zeropage.range.start,
+	ret = validate_range(ctx->mm, uffdio_zeropage.range.start,
 			     uffdio_zeropage.range.len);
 	if (ret)
 		goto out;

@@ -1815,7 +1813,7 @@ static int userfaultfd_writeprotect(struct userfaultfd_ctx *ctx,
 			   sizeof(struct uffdio_writeprotect)))
 		return -EFAULT;
 
-	ret = validate_range(ctx->mm, &uffdio_wp.range.start,
+	ret = validate_range(ctx->mm, uffdio_wp.range.start,
 			     uffdio_wp.range.len);
 	if (ret)
 		return ret;

@@ -1863,7 +1861,7 @@ static int userfaultfd_continue(struct userfaultfd_ctx *ctx, unsigned long arg)
 			   sizeof(uffdio_continue) - (sizeof(__s64))))
 		goto out;
 
-	ret = validate_range(ctx->mm, &uffdio_continue.range.start,
+	ret = validate_range(ctx->mm, uffdio_continue.range.start,
 			     uffdio_continue.range.len);
 	if (ret)
 		goto out;

-- 
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This test passes pointers obtained from anon_allocate_area to the
userfaultfd and mremap APIs. This causes a problem if the system
allocator returns tagged pointers because with the tagged address ABI
the kernel rejects tagged addresses passed to these APIs, which would
end up causing the test to fail. To make this test compatible with
such system allocators, stop using the system allocator to allocate
memory in anon_allocate_area, and instead just use mmap.

Co-developed-by: Lokesh Gidra <redacted>
Signed-off-by: Lokesh Gidra <redacted>
Signed-off-by: Peter Collingbourne <redacted>
Fixes: c47174fc362a ("userfaultfd: selftest")
Cc: <redacted> # 5.4
Link: https://linux-review.googlesource.com/id/Icac91064fcd923f77a83e8e133f8631c5b8fc241
---
 tools/testing/selftests/vm/userfaultfd.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/vm/userfaultfd.c b/tools/testing/selftests/vm/userfaultfd.c
index f5ab5e0312e7..d0f802053dfd 100644
--- a/tools/testing/selftests/vm/userfaultfd.c
+++ b/tools/testing/selftests/vm/userfaultfd.c

@@ -197,8 +197,10 @@ static int anon_release_pages(char *rel_area)
 
 static void anon_allocate_area(void **alloc_area)
 {
-	if (posix_memalign(alloc_area, page_size, nr_pages * page_size)) {
-		fprintf(stderr, "out of memory\n");
+	*alloc_area = mmap(NULL, nr_pages * page_size, PROT_READ | PROT_WRITE,
+			   MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
+	if (*alloc_area == MAP_FAILED) {
+		fprintf(stderr, "anon memory mmap failed\n");
 		*alloc_area = NULL;
 	}
 }

-- 
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On Wed, Jul 07, 2021 at 11:43:12AM -0700, Peter Collingbourne wrote:

If a user program uses userfaultfd on ranges of heap memory, it may
end up passing a tagged pointer to the kernel in the range.start
field of the UFFDIO_REGISTER ioctl. This can happen when using an
MTE-capable allocator, or on Android if using the Tagged Pointers
feature for MTE readiness [1].

When a fault subsequently occurs, the tag is stripped from the fault
address returned to the application in the fault.address field
of struct uffd_msg. However, from the application's perspective,
the tagged address *is* the memory address, so if the application
is unaware of memory tags, it may get confused by receiving an
address that is, from its point of view, outside of the bounds of the
allocation. We observed this behavior in the kselftest for userfaultfd
[2] but other applications could have the same problem.

Address this by not untagging pointers passed to the userfaultfd
ioctls. Instead, let the system call fail. This will provide an
early indication of problems with tag-unaware userspace code instead
of letting the code get confused later, and is consistent with how
we decided to handle brk/mmap/mremap in commit dcde237319e6 ("mm:
Avoid creating virtual address aliases in brk()/mmap()/mremap()"),
as well as being consistent with the existing tagged address ABI
documentation relating to how ioctl arguments are handled.

The code change is a revert of commit 7d0325749a6c ("userfaultfd:
untag user pointers") plus some fixups to some additional calls to
validate_range that have appeared since then.

[1] https://source.android.com/devices/tech/debug/tagged-pointers
[2] tools/testing/selftests/vm/userfaultfd.c

Signed-off-by: Peter Collingbourne <redacted>
Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
Link: https://linux-review.googlesource.com/id/I761aa9f0344454c482b83fcfcce547db0a25501b
Fixes: 63f0c6037965 ("arm64: Introduce prctl() options to control the tagged user addresses ABI")
Cc: <redacted> # 5.4

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

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On Wed, Jul 07, 2021 at 11:43:13AM -0700, Peter Collingbourne wrote:

This test passes pointers obtained from anon_allocate_area to the
userfaultfd and mremap APIs. This causes a problem if the system
allocator returns tagged pointers because with the tagged address ABI
the kernel rejects tagged addresses passed to these APIs, which would
end up causing the test to fail. To make this test compatible with
such system allocators, stop using the system allocator to allocate
memory in anon_allocate_area, and instead just use mmap.

Co-developed-by: Lokesh Gidra <redacted>
Signed-off-by: Lokesh Gidra <redacted>
Signed-off-by: Peter Collingbourne <redacted>
Fixes: c47174fc362a ("userfaultfd: selftest")
Cc: <redacted> # 5.4
Link: https://linux-review.googlesource.com/id/Icac91064fcd923f77a83e8e133f8631c5b8fc241

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

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On Wed,  7 Jul 2021 11:43:13 -0700 Peter Collingbourne [off-list ref] wrote:

This test passes pointers obtained from anon_allocate_area to the
userfaultfd and mremap APIs. This causes a problem if the system
allocator returns tagged pointers because with the tagged address ABI
the kernel rejects tagged addresses passed to these APIs, which would
end up causing the test to fail. To make this test compatible with
such system allocators, stop using the system allocator to allocate
memory in anon_allocate_area, and instead just use mmap.

Doesn't apply to current mainline.  Can you please resync and redo?

Co-developed-by: Lokesh Gidra <redacted>
Signed-off-by: Lokesh Gidra <redacted>
Signed-off-by: Peter Collingbourne <redacted>
Fixes: c47174fc362a ("userfaultfd: selftest")
Cc: <redacted> # 5.4

This patch then won't apply to earlier kenrels, so please be prepared
to help Greg resolve this.


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