From a34309d16f41c48ffd90e56a6f865d6a1a8c49f0 Mon Sep 17 00:00:00 2001

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

Date: Wed, 19 Feb 2020 12:31:56 +0000

Subject: [PATCH] mm: Avoid creating virtual address aliases in

 brk()/mmap()/mremap()

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@vger.kernel.org> # 5.4.x-

Cc: Andrew Morton <akpm@linux-foundation.org>

Cc: Florian Weimer <fweimer@redhat.com>

Reported-by: Victor Stinner <vstinner@redhat.com>

Acked-by: Will Deacon <will@kernel.org>

Acked-by: Andrey Konovalov <andreyknvl@google.com>

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

---

 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..f6289116893c 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 of aliasing 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 4390dbea4aa5..514cc19c5916 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;

-- 

2.24.1