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From a34309d16f41c48ffd90e56a6f865d6a1a8c49f0 Mon Sep 17 00:00:00 2001
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From: Catalin Marinas <catalin.marinas@arm.com>
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Date: Wed, 19 Feb 2020 12:31:56 +0000
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Subject: [PATCH] mm: Avoid creating virtual address aliases in
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 brk()/mmap()/mremap()
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Currently the arm64 kernel ignores the top address byte passed to brk(),
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mmap() and mremap(). When the user is not aware of the 56-bit address
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limit or relies on the kernel to return an error, untagging such
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pointers has the potential to create address aliases in user-space.
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Passing a tagged address to munmap(), madvise() is permitted since the
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tagged pointer is expected to be inside an existing mapping.
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The current behaviour breaks the existing glibc malloc() implementation
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which relies on brk() with an address beyond 56-bit to be rejected by
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the kernel.
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Remove untagging in the above functions by partially reverting commit
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ce18d171cb73 ("mm: untag user pointers in mmap/munmap/mremap/brk"). In
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addition, update the arm64 tagged-address-abi.rst document accordingly.
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Link: https://bugzilla.redhat.com/1797052
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Fixes: ce18d171cb73 ("mm: untag user pointers in mmap/munmap/mremap/brk")
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Cc: <stable@vger.kernel.org> # 5.4.x-
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Cc: Andrew Morton <akpm@linux-foundation.org>
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Cc: Florian Weimer <fweimer@redhat.com>
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Reported-by: Victor Stinner <vstinner@redhat.com>
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Acked-by: Will Deacon <will@kernel.org>
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Acked-by: Andrey Konovalov <andreyknvl@google.com>
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Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
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---
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 Documentation/arm64/tagged-address-abi.rst | 11 +++++++++--
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 mm/mmap.c                                  |  4 ----
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 mm/mremap.c                                |  1 -
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 3 files changed, 9 insertions(+), 7 deletions(-)
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diff --git a/Documentation/arm64/tagged-address-abi.rst b/Documentation/arm64/tagged-address-abi.rst
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index d4a85d535bf9..f6289116893c 100644
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--- a/Documentation/arm64/tagged-address-abi.rst
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+++ b/Documentation/arm64/tagged-address-abi.rst
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@@ -44,8 +44,15 @@ The AArch64 Tagged Address ABI has two stages of relaxation depending
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 how the user addresses are used by the kernel:
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 1. User addresses not accessed by the kernel but used for address space
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-   management (e.g. ``mmap()``, ``mprotect()``, ``madvise()``). The use
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-   of valid tagged pointers in this context is always allowed.
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+   management (e.g. ``mprotect()``, ``madvise()``). The use of valid
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+   tagged pointers in this context is allowed with the exception of
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+   ``brk()``, ``mmap()`` and the ``new_address`` argument to
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+   ``mremap()`` as these have the potential of aliasing with existing
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+   user addresses.
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+
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+   NOTE: This behaviour changed in v5.6 and so some earlier kernels may
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+   incorrectly accept valid tagged pointers for the ``brk()``,
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+   ``mmap()`` and ``mremap()`` system calls.
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 2. User addresses accessed by the kernel (e.g. ``write()``). This ABI
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    relaxation is disabled by default and the application thread needs to
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diff --git a/mm/mmap.c b/mm/mmap.c
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index 4390dbea4aa5..514cc19c5916 100644
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--- a/mm/mmap.c
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+++ b/mm/mmap.c
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@@ -195,8 +195,6 @@ SYSCALL_DEFINE1(brk, unsigned long, brk)
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 	bool downgraded = false;
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 	LIST_HEAD(uf);
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-	brk = untagged_addr(brk);
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-
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 	if (down_write_killable(&mm->mmap_sem))
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 		return -EINTR;
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@@ -1583,8 +1581,6 @@ unsigned long ksys_mmap_pgoff(unsigned long addr, unsigned long len,
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 	struct file *file = NULL;
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 	unsigned long retval;
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-	addr = untagged_addr(addr);
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-
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 	if (!(flags & MAP_ANONYMOUS)) {
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 		audit_mmap_fd(fd, flags);
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 		file = fget(fd);
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diff --git a/mm/mremap.c b/mm/mremap.c
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index 1fc8a29fbe3f..1d98281f7204 100644
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--- a/mm/mremap.c
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+++ b/mm/mremap.c
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@@ -607,7 +607,6 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
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 	LIST_HEAD(uf_unmap);
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 	addr = untagged_addr(addr);
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-	new_addr = untagged_addr(new_addr);
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 	if (flags & ~(MREMAP_FIXED | MREMAP_MAYMOVE))
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 		return ret;
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-- 
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2.24.1
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