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Privileged programs are supposed to be able to read uninitialized stack
memory (ever since 6715df8d5) but, before this patch, these accesses
were permitted inconsistently. In particular, accesses were permitted
above state->allocated_stack, but not below it. In other words, if the
stack was already "large enough", the access was permitted, but
otherwise the access was rejected instead of being allowed to "grow the
stack". This undesired rejection was happening in two places:
- in check_stack_slot_within_bounds()
- in check_stack_range_initialized()
This patch arranges for these accesses to be permitted. A bunch of tests
that were relying on the old rejection had to change; all of them were
changed to add also run unprivileged, in which case the old behavior
persists. One tests couldn't be updated - global_func16 - because it
can't run unprivileged for other reasons.
This patch also fixes the tracking of the stack size for variable-offset
reads. This second fix is bundled in the same commit as the first one
because they're inter-related. Before this patch, writes to the stack
using registers containing a variable offset (as opposed to registers
with fixed, known values) were not properly contributing to the
function's needed stack size. As a result, it was possible for a program
to verify, but then to attempt to read out-of-bounds data at runtime
because a too small stack had been allocated for it.
Each function tracks the size of the stack it needs in
bpf_subprog_info.stack_depth, which is maintained by
update_stack_depth(). For regular memory accesses, check_mem_access()
was calling update_state_depth() but it was passing in only the fixed
part of the offset register, ignoring the variable offset. This was
incorrect; the minimum possible value of that register should be used
instead.
This tracking is now fixed by centralizing the tracking of stack size in
grow_stack_state(), and by lifting the calls to grow_stack_state() to
check_stack_access_within_bounds() as suggested by Andrii. The code is
now simpler and more convincingly tracks the correct maximum stack size.
check_stack_range_initialized() can now rely on enough stack having been
allocated for the access; this helps with the fix for the first issue.
A few tests were changed to also check the stack depth computation. The
one that fails without this patch is verifier_var_off:stack_write_priv_vs_unpriv.
Fixes: 01f810ace9ed3 ("bpf: Allow variable-offset stack access")
Reported-by: Hao Sun <sunhao.th@gmail.com>
Signed-off-by: Andrei Matei <andreimatei1@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20231208032519.260451-3-andreimatei1@gmail.com
Closes: https://lore.kernel.org/bpf/CABWLsev9g8UP_c3a=1qbuZUi20tGoUXoU07FPf-5FLvhOKOY+Q@mail.gmail.com/
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Some verifier tests were missing F_NEEDS_EFFICIENT_UNALIGNED_ACCESS,
which made the test fail. Add the flag where needed.
Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230705113926.751791-2-bjorn@kernel.org
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Fix up unprivileged test case results for 'Dest pointer in r0' verifier tests
given they now need to reject R0 containing a pointer value, and add a couple
of new related ones with 32bit cmpxchg as well.
root@foo:~/bpf/tools/testing/selftests/bpf# ./test_verifier
#0/u invalid and of negative number OK
#0/p invalid and of negative number OK
[...]
#1268/p XDP pkt read, pkt_meta' <= pkt_data, bad access 1 OK
#1269/p XDP pkt read, pkt_meta' <= pkt_data, bad access 2 OK
#1270/p XDP pkt read, pkt_data <= pkt_meta', good access OK
#1271/p XDP pkt read, pkt_data <= pkt_meta', bad access 1 OK
#1272/p XDP pkt read, pkt_data <= pkt_meta', bad access 2 OK
Summary: 1900 PASSED, 0 SKIPPED, 0 FAILED
Acked-by: Brendan Jackman <jackmanb@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
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Test whether unprivileged would be able to leak the spilled pointer either
by exporting the returned value from the atomic{32,64} operation or by reading
and exporting the value from the stack after the atomic operation took place.
Note that for unprivileged, the below atomic cmpxchg test case named "Dest
pointer in r0 - succeed" is failing. The reason is that in the dst memory
location (r10 -8) there is the spilled register r10:
0: R1=ctx(id=0,off=0,imm=0) R10=fp0
0: (bf) r0 = r10
1: R0_w=fp0 R1=ctx(id=0,off=0,imm=0) R10=fp0
1: (7b) *(u64 *)(r10 -8) = r0
2: R0_w=fp0 R1=ctx(id=0,off=0,imm=0) R10=fp0 fp-8_w=fp
2: (b7) r1 = 0
3: R0_w=fp0 R1_w=invP0 R10=fp0 fp-8_w=fp
3: (db) r0 = atomic64_cmpxchg((u64 *)(r10 -8), r0, r1)
4: R0_w=fp0 R1_w=invP0 R10=fp0 fp-8_w=mmmmmmmm
4: (79) r1 = *(u64 *)(r0 -8)
5: R0_w=fp0 R1_w=invP(id=0) R10=fp0 fp-8_w=mmmmmmmm
5: (b7) r0 = 0
6: R0_w=invP0 R1_w=invP(id=0) R10=fp0 fp-8_w=mmmmmmmm
6: (95) exit
However, allowing this case for unprivileged is a bit useless given an
update with a new pointer will fail anyway:
0: R1=ctx(id=0,off=0,imm=0) R10=fp0
0: (bf) r0 = r10
1: R0_w=fp0 R1=ctx(id=0,off=0,imm=0) R10=fp0
1: (7b) *(u64 *)(r10 -8) = r0
2: R0_w=fp0 R1=ctx(id=0,off=0,imm=0) R10=fp0 fp-8_w=fp
2: (db) r0 = atomic64_cmpxchg((u64 *)(r10 -8), r0, r10)
R10 leaks addr into mem
Acked-by: Brendan Jackman <jackmanb@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
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Some new verifier tests that hit some important gaps in the parameter
space for atomic ops.
There are already exhaustive tests for the JIT part in
lib/test_bpf.c, but these exercise the verifier too.
Signed-off-by: Brendan Jackman <jackmanb@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211015093318.1273686-1-jackmanb@google.com
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As pointed out by Ilya and explained in the new comment, there's a
discrepancy between x86 and BPF CMPXCHG semantics: BPF always loads
the value from memory into r0, while x86 only does so when r0 and the
value in memory are different. The same issue affects s390.
At first this might sound like pure semantics, but it makes a real
difference when the comparison is 32-bit, since the load will
zero-extend r0/rax.
The fix is to explicitly zero-extend rax after doing such a
CMPXCHG. Since this problem affects multiple archs, this is done in
the verifier by patching in a BPF_ZEXT_REG instruction after every
32-bit cmpxchg. Any archs that don't need such manual zero-extension
can do a look-ahead with insn_is_zext to skip the unnecessary mov.
Note this still goes on top of Ilya's patch:
https://lore.kernel.org/bpf/20210301154019.129110-1-iii@linux.ibm.com/T/#u
Differences v5->v6[1]:
- Moved is_cmpxchg_insn and ensured it can be safely re-used. Also renamed it
and removed 'inline' to match the style of the is_*_function helpers.
- Fixed up comments in verifier test (thanks for the careful review, Martin!)
Differences v4->v5[1]:
- Moved the logic entirely into opt_subreg_zext_lo32_rnd_hi32, thanks to Martin
for suggesting this.
Differences v3->v4[1]:
- Moved the optimization against pointless zext into the correct place:
opt_subreg_zext_lo32_rnd_hi32 is called _after_ fixup_bpf_calls.
Differences v2->v3[1]:
- Moved patching into fixup_bpf_calls (patch incoming to rename this function)
- Added extra commentary on bpf_jit_needs_zext
- Added check to avoid adding a pointless zext(r0) if there's already one there.
Difference v1->v2[1]: Now solved centrally in the verifier instead of
specifically for the x86 JIT. Thanks to Ilya and Daniel for the suggestions!
[1] v5: https://lore.kernel.org/bpf/CA+i-1C3ytZz6FjcPmUg5s4L51pMQDxWcZNvM86w4RHZ_o2khwg@mail.gmail.com/T/#t
v4: https://lore.kernel.org/bpf/CA+i-1C3ytZz6FjcPmUg5s4L51pMQDxWcZNvM86w4RHZ_o2khwg@mail.gmail.com/T/#t
v3: https://lore.kernel.org/bpf/08669818-c99d-0d30-e1db-53160c063611@iogearbox.net/T/#t
v2: https://lore.kernel.org/bpf/08669818-c99d-0d30-e1db-53160c063611@iogearbox.net/T/#t
v1: https://lore.kernel.org/bpf/d7ebaefb-bfd6-a441-3ff2-2fdfe699b1d2@iogearbox.net/T/#t
Reported-by: Ilya Leoshkevich <iii@linux.ibm.com>
Fixes: 5ffa25502b5a ("bpf: Add instructions for atomic_[cmp]xchg")
Signed-off-by: Brendan Jackman <jackmanb@google.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Tested-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
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The prog_test that's added depends on Clang/LLVM features added by
Yonghong in commit 286daafd6512 (was https://reviews.llvm.org/D72184).
Note the use of a define called ENABLE_ATOMICS_TESTS: this is used
to:
- Avoid breaking the build for people on old versions of Clang
- Avoid needing separate lists of test objects for no_alu32, where
atomics are not supported even if Clang has the feature.
The atomics_test.o BPF object is built unconditionally both for
test_progs and test_progs-no_alu32. For test_progs, if Clang supports
atomics, ENABLE_ATOMICS_TESTS is defined, so it includes the proper
test code. Otherwise, progs and global vars are defined anyway, as
stubs; this means that the skeleton user code still builds.
The atomics_test.o userspace object is built once and used for both
test_progs and test_progs-no_alu32. A variable called skip_tests is
defined in the BPF object's data section, which tells the userspace
object whether to skip the atomics test.
Signed-off-by: Brendan Jackman <jackmanb@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210114181751.768687-11-jackmanb@google.com
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