tjh.dev
bpf: Fall back to nospec for Spectre v1
This implements the core of the series and causes the verifier to fall
back to mitigating Spectre v1 using speculation barriers. The approach
was presented at LPC'24 [1] and RAID'24 [2].
If we find any forbidden behavior on a speculative path, we insert a
nospec (e.g., lfence speculation barrier on x86) before the instruction
and stop verifying the path. While verifying a speculative path, we can
furthermore stop verification of that path whenever we encounter a
nospec instruction.
A minimal example program would look as follows:
A = true
B = true
if A goto e
f()
if B goto e
unsafe()
e: exit
There are the following speculative and non-speculative paths
(`cur->speculative` and `speculative` referring to the value of the
push_stack() parameters):
- A = true
- B = true
- if A goto e
- A && !cur->speculative && !speculative
- exit
- !A && !cur->speculative && speculative
- f()
- if B goto e
- B && cur->speculative && !speculative
- exit
- !B && cur->speculative && speculative
- unsafe()
If f() contains any unsafe behavior under Spectre v1 and the unsafe
behavior matches `state->speculative &&
error_recoverable_with_nospec(err)`, do_check() will now add a nospec
before f() instead of rejecting the program:
A = true
B = true
if A goto e
nospec
f()
if B goto e
unsafe()
e: exit
Alternatively, the algorithm also takes advantage of nospec instructions
inserted for other reasons (e.g., Spectre v4). Taking the program above
as an example, speculative path exploration can stop before f() if a
nospec was inserted there because of Spectre v4 sanitization.
In this example, all instructions after the nospec are dead code (and
with the nospec they are also dead code speculatively).
For this, it relies on the fact that speculation barriers generally
prevent all later instructions from executing if the speculation was not
correct:
* On Intel x86_64, lfence acts as full speculation barrier, not only as
a load fence [3]:
An LFENCE instruction or a serializing instruction will ensure that
no later instructions execute, even speculatively, until all prior
instructions complete locally. [...] Inserting an LFENCE instruction
after a bounds check prevents later operations from executing before
the bound check completes.
This was experimentally confirmed in [4].
* On AMD x86_64, lfence is dispatch-serializing [5] (requires MSR
C001_1029[1] to be set if the MSR is supported, this happens in
init_amd()). AMD further specifies "A dispatch serializing instruction
forces the processor to retire the serializing instruction and all
previous instructions before the next instruction is executed" [8]. As
dispatch is not specific to memory loads or branches, lfence therefore
also affects all instructions there. Also, if retiring a branch means
it's PC change becomes architectural (should be), this means any
"wrong" speculation is aborted as required for this series.
* ARM's SB speculation barrier instruction also affects "any instruction
that appears later in the program order than the barrier" [6].
* PowerPC's barrier also affects all subsequent instructions [7]:
[...] executing an ori R31,R31,0 instruction ensures that all
instructions preceding the ori R31,R31,0 instruction have completed
before the ori R31,R31,0 instruction completes, and that no
subsequent instructions are initiated, even out-of-order, until
after the ori R31,R31,0 instruction completes. The ori R31,R31,0
instruction may complete before storage accesses associated with
instructions preceding the ori R31,R31,0 instruction have been
performed
Regarding the example, this implies that `if B goto e` will not execute
before `if A goto e` completes. Once `if A goto e` completes, the CPU
should find that the speculation was wrong and continue with `exit`.
If there is any other path that leads to `if B goto e` (and therefore
`unsafe()`) without going through `if A goto e`, then a nospec will
still be needed there. However, this patch assumes this other path will
be explored separately and therefore be discovered by the verifier even
if the exploration discussed here stops at the nospec.
This patch furthermore has the unfortunate consequence that Spectre v1
mitigations now only support architectures which implement BPF_NOSPEC.
Before this commit, Spectre v1 mitigations prevented exploits by
rejecting the programs on all architectures. Because some JITs do not
implement BPF_NOSPEC, this patch therefore may regress unpriv BPF's
security to a limited extent:
* The regression is limited to systems vulnerable to Spectre v1, have
unprivileged BPF enabled, and do NOT emit insns for BPF_NOSPEC. The
latter is not the case for x86 64- and 32-bit, arm64, and powerpc
64-bit and they are therefore not affected by the regression.
According to commit a6f6a95f2580 ("LoongArch, bpf: Fix jit to skip
speculation barrier opcode"), LoongArch is not vulnerable to Spectre
v1 and therefore also not affected by the regression.
* To the best of my knowledge this regression may therefore only affect
MIPS. This is deemed acceptable because unpriv BPF is still disabled
there by default. As stated in a previous commit, BPF_NOSPEC could be
implemented for MIPS based on GCC's speculation_barrier
implementation.
* It is unclear which other architectures (besides x86 64- and 32-bit,
ARM64, PowerPC 64-bit, LoongArch, and MIPS) supported by the kernel
are vulnerable to Spectre v1. Also, it is not clear if barriers are
available on these architectures. Implementing BPF_NOSPEC on these
architectures therefore is non-trivial. Searching GCC and the kernel
for speculation barrier implementations for these architectures
yielded no result.
* If any of those regressed systems is also vulnerable to Spectre v4,
the system was already vulnerable to Spectre v4 attacks based on
unpriv BPF before this patch and the impact is therefore further
limited.
As an alternative to regressing security, one could still reject
programs if the architecture does not emit BPF_NOSPEC (e.g., by removing
the empty BPF_NOSPEC-case from all JITs except for LoongArch where it
appears justified). However, this will cause rejections on these archs
that are likely unfounded in the vast majority of cases.
In the tests, some are now successful where we previously had a
false-positive (i.e., rejection). Change them to reflect where the
nospec should be inserted (using __xlated_unpriv) and modify the error
message if the nospec is able to mitigate a problem that previously
shadowed another problem (in that case __xlated_unpriv does not work,
therefore just add a comment).
Define SPEC_V1 to avoid duplicating this ifdef whenever we check for
nospec insns using __xlated_unpriv, define it here once. This also
improves readability. PowerPC can probably also be added here. However,
omit it for now because the BPF CI currently does not include a test.
Limit it to EPERM, EACCES, and EINVAL (and not everything except for
EFAULT and ENOMEM) as it already has the desired effect for most
real-world programs. Briefly went through all the occurrences of EPERM,
EINVAL, and EACCESS in verifier.c to validate that catching them like
this makes sense.
Thanks to Dustin for their help in checking the vendor documentation.
[1] https://lpc.events/event/18/contributions/1954/ ("Mitigating
Spectre-PHT using Speculation Barriers in Linux eBPF")
[2] https://arxiv.org/pdf/2405.00078 ("VeriFence: Lightweight and
Precise Spectre Defenses for Untrusted Linux Kernel Extensions")
[3] https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/technical-documentation/runtime-speculative-side-channel-mitigations.html
("Managed Runtime Speculative Execution Side Channel Mitigations")
[4] https://dl.acm.org/doi/pdf/10.1145/3359789.3359837 ("Speculator: a
tool to analyze speculative execution attacks and mitigations" -
Section 4.6 "Stopping Speculative Execution")
[5] https://www.amd.com/content/dam/amd/en/documents/processor-tech-docs/programmer-references/software-techniques-for-managing-speculation.pdf
("White Paper - SOFTWARE TECHNIQUES FOR MANAGING SPECULATION ON AMD
PROCESSORS - REVISION 5.09.23")
[6] https://developer.arm.com/documentation/ddi0597/2020-12/Base-Instructions/SB--Speculation-Barrier-
("SB - Speculation Barrier - Arm Armv8-A A32/T32 Instruction Set
Architecture (2020-12)")
[7] https://wiki.raptorcs.com/w/images/5/5f/OPF_PowerISA_v3.1C.pdf
("Power ISA™ - Version 3.1C - May 26, 2024 - Section 9.2.1 of Book
III")
[8] https://www.amd.com/content/dam/amd/en/documents/processor-tech-docs/programmer-references/40332.pdf
("AMD64 Architecture Programmer’s Manual Volumes 1–5 - Revision 4.08
- April 2024 - 7.6.4 Serializing Instructions")
Signed-off-by: Luis Gerhorst <luis.gerhorst@fau.de>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Acked-by: Henriette Herzog <henriette.herzog@rub.de>
Cc: Dustin Nguyen <nguyen@cs.fau.de>
Cc: Maximilian Ott <ott@cs.fau.de>
Cc: Milan Stephan <milan.stephan@fau.de>
Link: https://lore.kernel.org/r/20250603212428.338473-1-luis.gerhorst@fau.de
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
authored by