[PW_SID:1060691] Fix bugs and performance of kstack offset randomisation#1543
[PW_SID:1060691] Fix bugs and performance of kstack offset randomisation#1543linux-riscv-bot wants to merge 2 commits into
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kstack_offset was previously maintained per-cpu, but this caused a couple of issues. So let's instead make it per-task. Issue 1: add_random_kstack_offset() and choose_random_kstack_offset() expected and required to be called with interrupts and preemption disabled so that it could manipulate per-cpu state. But arm64, loongarch and risc-v are calling them with interrupts and preemption enabled. I don't _think_ this causes any functional issues, but it's certainly unexpected and could lead to manipulating the wrong cpu's state, which could cause a minor performance degradation due to bouncing the cache lines. By maintaining the state per-task those functions can safely be called in preemptible context. Issue 2: add_random_kstack_offset() is called before executing the syscall and expands the stack using a previously chosen random offset. choose_random_kstack_offset() is called after executing the syscall and chooses and stores a new random offset for the next syscall. With per-cpu storage for this offset, an attacker could force cpu migration during the execution of the syscall and prevent the offset from being updated for the original cpu such that it is predictable for the next syscall on that cpu. By maintaining the state per-task, this problem goes away because the per-task random offset is updated after the syscall regardless of which cpu it is executing on. Fixes: 39218ff ("stack: Optionally randomize kernel stack offset each syscall") Closes: https://lore.kernel.org/all/dd8c37bc-795f-4c7a-9086-69e584d8ab24@arm.com/ Cc: stable@vger.kernel.org Acked-by: Mark Rutland <mark.rutland@arm.com> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com> Signed-off-by: Linux RISC-V bot <linux.riscv.bot@gmail.com>
Previously different architectures were using random sources of
differing strength and cost to decide the random kstack offset. A number
of architectures (loongarch, powerpc, s390, x86) were using their
timestamp counter, at whatever the frequency happened to be. Other
arches (arm64, riscv) were using entropy from the crng via
get_random_u16().
There have been concerns that in some cases the timestamp counters may
be too weak, because they can be easily guessed or influenced by user
space. And get_random_u16() has been shown to be too costly for the
level of protection kstack offset randomization provides.
So let's use a common, architecture-agnostic source of entropy; a
per-cpu prng, seeded at boot-time from the crng. This has a few
benefits:
- We can remove choose_random_kstack_offset(); That was only there to
try to make the timestamp counter value a bit harder to influence
from user space [*].
- The architecture code is simplified. All it has to do now is call
add_random_kstack_offset() in the syscall path.
- The strength of the randomness can be reasoned about independently
of the architecture.
- Arches previously using get_random_u16() now have much faster
syscall paths, see below results.
[*] Additionally, this gets rid of some redundant work on s390 and x86.
Before this patch, those architectures called
choose_random_kstack_offset() under arch_exit_to_user_mode_prepare(),
which is also called for exception returns to userspace which were *not*
syscalls (e.g. regular interrupts). Getting rid of
choose_random_kstack_offset() avoids a small amount of redundant work
for the non-syscall cases.
In some configurations, add_random_kstack_offset() will now call
instrumentable code, so for a couple of arches, I have moved the call a
bit later to the first point where instrumentation is allowed. This
doesn't impact the efficacy of the mechanism.
There have been some claims that a prng may be less strong than the
timestamp counter if not regularly reseeded. But the prng has a period
of about 2^113. So as long as the prng state remains secret, it should
not be possible to guess. If the prng state can be accessed, we have
bigger problems.
Additionally, we are only consuming 6 bits to randomize the stack, so
there are only 64 possible random offsets. I assert that it would be
trivial for an attacker to brute force by repeating their attack and
waiting for the random stack offset to be the desired one. The prng
approach seems entirely proportional to this level of protection.
Performance data are provided below. The baseline is v6.18 with rndstack
on for each respective arch. (I)/(R) indicate statistically significant
improvement/regression. arm64 platform is AWS Graviton3 (m7g.metal).
x86_64 platform is AWS Sapphire Rapids (m7i.24xlarge):
+-----------------+--------------+---------------+---------------+
| Benchmark | Result Class | per-cpu-prng | per-cpu-prng |
| | | arm64 (metal) | x86_64 (VM) |
+=================+==============+===============+===============+
| syscall/getpid | mean (ns) | (I) -9.50% | (I) -17.65% |
| | p99 (ns) | (I) -59.24% | (I) -24.41% |
| | p99.9 (ns) | (I) -59.52% | (I) -28.52% |
+-----------------+--------------+---------------+---------------+
| syscall/getppid | mean (ns) | (I) -9.52% | (I) -19.24% |
| | p99 (ns) | (I) -59.25% | (I) -25.03% |
| | p99.9 (ns) | (I) -59.50% | (I) -28.17% |
+-----------------+--------------+---------------+---------------+
| syscall/invalid | mean (ns) | (I) -10.31% | (I) -18.56% |
| | p99 (ns) | (I) -60.79% | (I) -20.06% |
| | p99.9 (ns) | (I) -61.04% | (I) -25.04% |
+-----------------+--------------+---------------+---------------+
I tested an earlier version of this change on x86 bare metal and it
showed a smaller but still significant improvement. The bare metal
system wasn't available this time around so testing was done in a VM
instance. I'm guessing the cost of rdtsc is higher for VMs.
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: Linux RISC-V bot <linux.riscv.bot@gmail.com>
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Patch 1: "[v5,1/2] randomize_kstack: Maintain kstack_offset per task" |
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Patch 1: "[v5,1/2] randomize_kstack: Maintain kstack_offset per task" |
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Patch 1: "[v5,1/2] randomize_kstack: Maintain kstack_offset per task" |
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Patch 1: "[v5,1/2] randomize_kstack: Maintain kstack_offset per task" |
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Patch 1: "[v5,1/2] randomize_kstack: Maintain kstack_offset per task" |
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Patch 1: "[v5,1/2] randomize_kstack: Maintain kstack_offset per task" |
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Patch 1: "[v5,1/2] randomize_kstack: Maintain kstack_offset per task" |
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Patch 1: "[v5,1/2] randomize_kstack: Maintain kstack_offset per task" |
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Patch 1: "[v5,1/2] randomize_kstack: Maintain kstack_offset per task" |
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Patch 1: "[v5,1/2] randomize_kstack: Maintain kstack_offset per task" |
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Patch 1: "[v5,1/2] randomize_kstack: Maintain kstack_offset per task" |
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Patch 1: "[v5,1/2] randomize_kstack: Maintain kstack_offset per task" |
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Patch 2: "[v5,2/2] randomize_kstack: Unify random source across arches" |
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Patch 2: "[v5,2/2] randomize_kstack: Unify random source across arches" |
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Patch 2: "[v5,2/2] randomize_kstack: Unify random source across arches" |
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Patch 2: "[v5,2/2] randomize_kstack: Unify random source across arches" |
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Patch 2: "[v5,2/2] randomize_kstack: Unify random source across arches" |
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Patch 2: "[v5,2/2] randomize_kstack: Unify random source across arches" |
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Patch 2: "[v5,2/2] randomize_kstack: Unify random source across arches" |
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Patch 2: "[v5,2/2] randomize_kstack: Unify random source across arches" |
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Patch 2: "[v5,2/2] randomize_kstack: Unify random source across arches" |
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Patch 2: "[v5,2/2] randomize_kstack: Unify random source across arches" |
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Patch 2: "[v5,2/2] randomize_kstack: Unify random source across arches" |
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Patch 2: "[v5,2/2] randomize_kstack: Unify random source across arches" |
2 participants
PR for series 1060691 applied to workflow__riscv__fixes
Name: Fix bugs and performance of kstack offset randomisation
URL: https://patchwork.kernel.org/project/linux-riscv/list/?series=1060691
Version: 5