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Date: Tue, 12 Mar 2019 20:30:06 -0500
From: Ali Saidi <asaidi@...il.com>
To: oss-security@...ts.openwall.com
Cc: alisaidi@...zon.com, "Liguori, Anthony" <aliguori@...zon.com>, dwmw@...zon.co.uk,
pzb@...zon.com
Subject: Stack/Heap Clashing on Linux >=4.13 when loader directly invoked
Resending due to the original being dropped...
Out of an abundance of caution this kernel issue was pre-disclosed with a
suggested patch to linux-distros@ and a patch has now been sent to lkml (
https://patchwork.kernel.org/project/linux-arm-kernel/list/?series=90691).
In Linux 4.13 a change was committed that special cased the kernel ELF
loader when the loader is invoked directly
(eab09532d40090698b05a07c1c87f39fdbc5fab5; binfmt_elf: use ELF_ET_DYN_BASE
only for PIE). Generally, the loader isn’t invoked directly and this issue
is limited to cases where it is, (e.g to set a non-inheritable
LD_LIBRARY_PATH, testing new versions of the loader). While this issue is
found on an arm64 system, the issue exists for other architectures as well,
with less frequency, and was observed occurring for both arm64 and x86_64.
When ELF binary is loaded directly, the kernel loader places it at
ELF_ET_DYN_BASE so there is a large separation between the stack and the
heap. However, when the loader is run directly, the kernel portion of the
loader sets load_bias = 0 which implies that mmap picks the address to load
it at. It does this by picking an address no higher than mm->mmap_base.
This address is calculated by starting at STACK_TOP and subtracting the
stack size, the stack randomization, and mmap_rnd_bits on arm64.
----> STACK_TOP
----> STACK_RND (-1GB)
----> stack_guard_gap (-256KB)
----> mmap_base (-[0-1GB])
If heap randomization is enabled (randomize_va_space = 2) the kernel loader
will call arch_randomize_brk() to also randomize the offset of the brk from
the heap. On arm64 this adds up to 1GB of randomization as the default
setting for mmap_rnd_bits is 1GB. Depending on the random offsets generated
for the heap and stack we can end up with a situation where the stack
randomization places the stack relatively far down in its region, the mmap
randomization is relatively small, and the brk randomization is large
leading to the stack and heap being arbitrarily close.
----> STACK_TOP (0x1000000000000)
----> bottom of stack (0xffffc067c4f0)
----> top of heap (0xffffc067c000)
----> STACK_RND (0xffffc0000000)
----> mmap_base (-0GB])
arm64 and x86 appear to do roughly the same thing here. Invoking a program
via ld-linux.so directly on arm64 surfaces the issue after a few thousand
invocations. On x86 arch_randomize_brk() is smaller (32MB) and the default
setting for mmap_rnd_bits defaults to a 1TB which makes the situation much
less likely to occur, but it’s still possible. The same should hold for
other architectures but I haven’t confirmed it.
Ali
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