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Subject[PATCH 4/6] seccomp: Emulate basic filters for constant action results
Date
This emulates absolutely the most basic seccomp filters to figure out
if they will always give the same results for a given arch/nr combo.

Nearly all seccomp filters are built from the following ops:

BPF_LD | BPF_W | BPF_ABS
BPF_JMP | BPF_JEQ | BPF_K
BPF_JMP | BPF_JGE | BPF_K
BPF_JMP | BPF_JGT | BPF_K
BPF_JMP | BPF_JSET | BPF_K
BPF_JMP | BPF_JA
BPF_RET | BPF_K

These are now emulated to check for accesses beyond seccomp_data::arch
or unknown instructions.

Not yet implemented are:

BPF_ALU | BPF_AND (generated by libseccomp and Chrome)

Suggested-by: Jann Horn <jannh@google.com>
Link: https://lore.kernel.org/lkml/CAG48ez1p=dR_2ikKq=xVxkoGg0fYpTBpkhJSv1w-6BG=76PAvw@mail.gmail.com/
Signed-off-by: Kees Cook <keescook@chromium.org>
---
kernel/seccomp.c | 82 ++++++++++++++++++++++++++++++++++++++++++++---
net/core/filter.c | 3 +-
2 files changed, 79 insertions(+), 6 deletions(-)

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 111a238bc532..9921f6f39d12 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -610,7 +610,12 @@ static struct seccomp_filter *seccomp_prepare_filter(struct sock_fprog *fprog)
{
struct seccomp_filter *sfilter;
int ret;
- const bool save_orig = IS_ENABLED(CONFIG_CHECKPOINT_RESTORE);
+ const bool save_orig =
+#if defined(CONFIG_CHECKPOINT_RESTORE) || defined(SECCOMP_ARCH)
+ true;
+#else
+ false;
+#endif

if (fprog->len == 0 || fprog->len > BPF_MAXINSNS)
return ERR_PTR(-EINVAL);
@@ -690,11 +695,78 @@ static inline bool sd_touched(pte_t *ptep)
* This approach could also be used to test for access to sd->arch too,
* if we wanted to warn about compat-unsafe filters.
*/
-static inline bool seccomp_filter_action_is_constant(struct bpf_prog *prog,
- struct seccomp_data *sd,
- u32 *action)
+static bool seccomp_filter_action_is_constant(struct bpf_prog *prog,
+ struct seccomp_data *sd,
+ u32 *action)
{
- /* No evaluation implementation yet. */
+ struct sock_fprog_kern *fprog = prog->orig_prog;
+ unsigned int insns;
+ unsigned int reg_value = 0;
+ unsigned int pc;
+ bool op_res;
+
+ if (WARN_ON_ONCE(!fprog))
+ return false;
+
+ insns = bpf_classic_proglen(fprog);
+ for (pc = 0; pc < insns; pc++) {
+ struct sock_filter *insn = &fprog->filter[pc];
+ u16 code = insn->code;
+ u32 k = insn->k;
+
+ switch (code) {
+ case BPF_LD | BPF_W | BPF_ABS:
+ switch (k) {
+ case offsetof(struct seccomp_data, nr):
+ reg_value = sd->nr;
+ break;
+ case offsetof(struct seccomp_data, arch):
+ reg_value = sd->arch;
+ break;
+ default:
+ /* can't optimize (non-constant value load) */
+ return false;
+ }
+ break;
+ case BPF_RET | BPF_K:
+ *action = insn->k;
+ /* success: reached return with constant values only */
+ return true;
+ case BPF_JMP | BPF_JA:
+ pc += insn->k;
+ break;
+ case BPF_JMP | BPF_JEQ | BPF_K:
+ case BPF_JMP | BPF_JGE | BPF_K:
+ case BPF_JMP | BPF_JGT | BPF_K:
+ case BPF_JMP | BPF_JSET | BPF_K:
+ switch (BPF_OP(code)) {
+ case BPF_JEQ:
+ op_res = reg_value == k;
+ break;
+ case BPF_JGE:
+ op_res = reg_value >= k;
+ break;
+ case BPF_JGT:
+ op_res = reg_value > k;
+ break;
+ case BPF_JSET:
+ op_res = !!(reg_value & k);
+ break;
+ default:
+ /* can't optimize (unknown jump) */
+ return false;
+ }
+
+ pc += op_res ? insn->jt : insn->jf;
+ break;
+ default:
+ /* can't optimize (unknown insn) */
+ return false;
+ }
+ }
+
+ /* ran off the end of the filter?! */
+ WARN_ON(1);
return false;
}

diff --git a/net/core/filter.c b/net/core/filter.c
index b2df52086445..cb1bdb0bfe87 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -1145,7 +1145,7 @@ static int bpf_prog_store_orig_filter(struct bpf_prog *fp,
return 0;
}

-static void bpf_release_orig_filter(struct bpf_prog *fp)
+void bpf_release_orig_filter(struct bpf_prog *fp)
{
struct sock_fprog_kern *fprog = fp->orig_prog;

@@ -1154,6 +1154,7 @@ static void bpf_release_orig_filter(struct bpf_prog *fp)
kfree(fprog);
}
}
+EXPORT_SYMBOL_GPL(bpf_release_orig_filter);

static void __bpf_prog_release(struct bpf_prog *prog)
{
--
2.25.1
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