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mm: mmap: allow for the maximum number of bits for randomizing mmap_base by default

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An ASLR regression was noticed [1] and tracked down to file-mapped areas
being backed by THP in recent kernels.  The 21-bit alignment constraint
for such mappings reduces the entropy for randomizing the placement of
64-bit library mappings and breaks ASLR completely for 32-bit libraries.

The reported issue is easily addressed by increasing vm.mmap_rnd_bits and
vm.mmap_rnd_compat_bits.  This patch just provides a simple way to set
ARCH_MMAP_RND_BITS and ARCH_MMAP_RND_COMPAT_BITS to their maximum values
allowed by the architecture at build time.

[1] https://zolutal.github.io/aslrnt/

[akpm@linux-foundation.org: default to `y' if 32-bit, per Rafael]
Link: https://lkml.kernel.org/r/20240606180622.102099-1-aquini@redhat.com
Fixes: 1854bc6e24 ("mm/readahead: Align file mappings for non-DAX")
Signed-off-by: Rafael Aquini <aquini@redhat.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Samuel Holland <samuel.holland@sifive.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Rafael Aquini 2024-06-06 14:06:22 -04:00 committed by Andrew Morton
parent c1558bc57b
commit 3afb76a66b
1 changed files with 12 additions and 0 deletions
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12
arch/Kconfig
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@ -1046,10 +1046,21 @@ config ARCH_MMAP_RND_BITS_MAX
config ARCH_MMAP_RND_BITS_DEFAULT
int
config FORCE_MAX_MMAP_RND_BITS
bool "Force maximum number of bits to use for ASLR of mmap base address"
default y if !64BIT
help
ARCH_MMAP_RND_BITS and ARCH_MMAP_RND_COMPAT_BITS represent the number
of bits to use for ASLR and if no custom value is assigned (EXPERT)
then the architecture's lower bound (minimum) value is assumed.
This toggle changes that default assumption to assume the arch upper
bound (maximum) value instead.
config ARCH_MMAP_RND_BITS
int "Number of bits to use for ASLR of mmap base address" if EXPERT
range ARCH_MMAP_RND_BITS_MIN ARCH_MMAP_RND_BITS_MAX
default ARCH_MMAP_RND_BITS_DEFAULT if ARCH_MMAP_RND_BITS_DEFAULT
default ARCH_MMAP_RND_BITS_MAX if FORCE_MAX_MMAP_RND_BITS
default ARCH_MMAP_RND_BITS_MIN
depends on HAVE_ARCH_MMAP_RND_BITS
help
@ -1084,6 +1095,7 @@ config ARCH_MMAP_RND_COMPAT_BITS
int "Number of bits to use for ASLR of mmap base address for compatible applications" if EXPERT
range ARCH_MMAP_RND_COMPAT_BITS_MIN ARCH_MMAP_RND_COMPAT_BITS_MAX
default ARCH_MMAP_RND_COMPAT_BITS_DEFAULT if ARCH_MMAP_RND_COMPAT_BITS_DEFAULT
default ARCH_MMAP_RND_COMPAT_BITS_MAX if FORCE_MAX_MMAP_RND_BITS
default ARCH_MMAP_RND_COMPAT_BITS_MIN
depends on HAVE_ARCH_MMAP_RND_COMPAT_BITS
help