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

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: 1854bc6e2420 ("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>

authored by

Rafael Aquini and committed by
Andrew Morton 3afb76a6 c1558bc5

+12
+12
arch/Kconfig
··· 1046 1046 config ARCH_MMAP_RND_BITS_DEFAULT 1047 1047 int 1048 1048 1049 + config FORCE_MAX_MMAP_RND_BITS 1050 + bool "Force maximum number of bits to use for ASLR of mmap base address" 1051 + default y if !64BIT 1052 + help 1053 + ARCH_MMAP_RND_BITS and ARCH_MMAP_RND_COMPAT_BITS represent the number 1054 + of bits to use for ASLR and if no custom value is assigned (EXPERT) 1055 + then the architecture's lower bound (minimum) value is assumed. 1056 + This toggle changes that default assumption to assume the arch upper 1057 + bound (maximum) value instead. 1058 + 1049 1059 config ARCH_MMAP_RND_BITS 1050 1060 int "Number of bits to use for ASLR of mmap base address" if EXPERT 1051 1061 range ARCH_MMAP_RND_BITS_MIN ARCH_MMAP_RND_BITS_MAX 1052 1062 default ARCH_MMAP_RND_BITS_DEFAULT if ARCH_MMAP_RND_BITS_DEFAULT 1063 + default ARCH_MMAP_RND_BITS_MAX if FORCE_MAX_MMAP_RND_BITS 1053 1064 default ARCH_MMAP_RND_BITS_MIN 1054 1065 depends on HAVE_ARCH_MMAP_RND_BITS 1055 1066 help ··· 1095 1084 int "Number of bits to use for ASLR of mmap base address for compatible applications" if EXPERT 1096 1085 range ARCH_MMAP_RND_COMPAT_BITS_MIN ARCH_MMAP_RND_COMPAT_BITS_MAX 1097 1086 default ARCH_MMAP_RND_COMPAT_BITS_DEFAULT if ARCH_MMAP_RND_COMPAT_BITS_DEFAULT 1087 + default ARCH_MMAP_RND_COMPAT_BITS_MAX if FORCE_MAX_MMAP_RND_BITS 1098 1088 default ARCH_MMAP_RND_COMPAT_BITS_MIN 1099 1089 depends on HAVE_ARCH_MMAP_RND_COMPAT_BITS 1100 1090 help