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authorDaniel Borkmann <daniel@iogearbox.net>2021-12-07 14:02:02 +0300
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2021-12-22 11:32:35 +0300
commitf87a6c160ecc8c7b417d25f508d3f076fe346136 (patch)
tree2535725546f1888d39e4c274983121082666de6c /kernel
parentd0d68083f273525ebae48996d196a706232b2d84 (diff)
downloadlinux-f87a6c160ecc8c7b417d25f508d3f076fe346136.tar.xz
bpf: Fix kernel address leakage in atomic cmpxchg's r0 aux reg
commit a82fe085f344ef20b452cd5f481010ff96b5c4cd upstream. The implementation of BPF_CMPXCHG on a high level has the following parameters: .-[old-val] .-[new-val] BPF_R0 = cmpxchg{32,64}(DST_REG + insn->off, BPF_R0, SRC_REG) `-[mem-loc] `-[old-val] Given a BPF insn can only have two registers (dst, src), the R0 is fixed and used as an auxilliary register for input (old value) as well as output (returning old value from memory location). While the verifier performs a number of safety checks, it misses to reject unprivileged programs where R0 contains a pointer as old value. Through brute-forcing it takes about ~16sec on my machine to leak a kernel pointer with BPF_CMPXCHG. The PoC is basically probing for kernel addresses by storing the guessed address into the map slot as a scalar, and using the map value pointer as R0 while SRC_REG has a canary value to detect a matching address. Fix it by checking R0 for pointers, and reject if that's the case for unprivileged programs. Fixes: 5ffa25502b5a ("bpf: Add instructions for atomic_[cmp]xchg") Reported-by: Ryota Shiga (Flatt Security) Acked-by: Brendan Jackman <jackmanb@google.com> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net> Signed-off-by: Alexei Starovoitov <ast@kernel.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Diffstat (limited to 'kernel')
-rw-r--r--kernel/bpf/verifier.c9
1 files changed, 8 insertions, 1 deletions
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 6ede2e4e2fdd..93c3a332e853 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -4386,9 +4386,16 @@ static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_i
if (insn->imm == BPF_CMPXCHG) {
/* Check comparison of R0 with memory location */
- err = check_reg_arg(env, BPF_REG_0, SRC_OP);
+ const u32 aux_reg = BPF_REG_0;
+
+ err = check_reg_arg(env, aux_reg, SRC_OP);
if (err)
return err;
+
+ if (is_pointer_value(env, aux_reg)) {
+ verbose(env, "R%d leaks addr into mem\n", aux_reg);
+ return -EACCES;
+ }
}
if (is_pointer_value(env, insn->src_reg)) {