[Focal] mm: Avoid creating virtual address aliases in brk()/mmap()/mremap()

[Thadeu Lima de Souza Cascardo]

From: Catalin Marinas <catalin.marinas at arm.com>

CVE-2020-9391

Currently the arm64 kernel ignores the top address byte passed to brk(),
mmap() and mremap(). When the user is not aware of the 56-bit address
limit or relies on the kernel to return an error, untagging such
pointers has the potential to create address aliases in user-space.
Passing a tagged address to munmap(), madvise() is permitted since the
tagged pointer is expected to be inside an existing mapping.

The current behaviour breaks the existing glibc malloc() implementation
which relies on brk() with an address beyond 56-bit to be rejected by
the kernel.

Remove untagging in the above functions by partially reverting commit
ce18d171cb73 ("mm: untag user pointers in mmap/munmap/mremap/brk"). In
addition, update the arm64 tagged-address-abi.rst document accordingly.

Link: https://bugzilla.redhat.com/1797052
Fixes: ce18d171cb73 ("mm: untag user pointers in mmap/munmap/mremap/brk")
Cc: <stable at vger.kernel.org> # 5.4.x-
Cc: Florian Weimer <fweimer at redhat.com>
Reviewed-by: Andrew Morton <akpm at linux-foundation.org>
Reported-by: Victor Stinner <vstinner at redhat.com>
Acked-by: Will Deacon <will at kernel.org>
Acked-by: Andrey Konovalov <andreyknvl at google.com>
Signed-off-by: Catalin Marinas <catalin.marinas at arm.com>
Signed-off-by: Will Deacon <will at kernel.org>
(cherry picked from commit dcde237319e626d1ec3c9d8b7613032f0fd4663a)
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
---

This has been built-tested on arm64.

---
 Documentation/arm64/tagged-address-abi.rst | 11 +++++++++--
 mm/mmap.c                                  |  4 ----
 mm/mremap.c                                |  1 -
 3 files changed, 9 insertions(+), 7 deletions(-)

diff --git a/Documentation/arm64/tagged-address-abi.rst b/Documentation/arm64/tagged-address-abi.rst
index d4a85d535bf9..4a9d9c794ee5 100644
--- a/Documentation/arm64/tagged-address-abi.rst
+++ b/Documentation/arm64/tagged-address-abi.rst
@@ -44,8 +44,15 @@ The AArch64 Tagged Address ABI has two stages of relaxation depending
 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. ``mmap()``, ``mprotect()``, ``madvise()``). The use
-   of valid tagged pointers in this context is always allowed.
+   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.
 
 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/mm/mmap.c b/mm/mmap.c
index e697082d36e4..68aed9b1a7f5 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -195,8 +195,6 @@ SYSCALL_DEFINE1(brk, unsigned long, brk)
 	bool downgraded = false;
 	LIST_HEAD(uf);
 
-	brk = untagged_addr(brk);
-
 	if (down_write_killable(&mm->mmap_sem))
 		return -EINTR;
 
@@ -1583,8 +1581,6 @@ unsigned long ksys_mmap_pgoff(unsigned long addr, unsigned long len,
 	struct file *file = NULL;
 	unsigned long retval;
 
-	addr = untagged_addr(addr);
-
 	if (!(flags & MAP_ANONYMOUS)) {
 		audit_mmap_fd(fd, flags);
 		file = fget(fd);
diff --git a/mm/mremap.c b/mm/mremap.c
index 1fc8a29fbe3f..1d98281f7204 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -607,7 +607,6 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
 	LIST_HEAD(uf_unmap);
 
 	addr = untagged_addr(addr);
-	new_addr = untagged_addr(new_addr);
 
 	if (flags & ~(MREMAP_FIXED | MREMAP_MAYMOVE))
 		return ret;
-- 
2.25.1