bpf: fix stacksafe exploration when comparing states

Commit cc2b14d51053 ("bpf: teach verifier to recognize zero initialized
stack") introduced a very relaxed check when comparing stacks of different
states, effectively returning a positive result in many cases where it
shouldn't.

This can create problems in cases such as this following C pseudocode:

long var;
long *x = bpf_map_lookup(...);
if (!x)
        return;

if (*x != 0xbeef)
        var = 0;
else
        var = 1;

/* This is the key part, calling a helper causes an explored state
 * to be saved with the information that "var" is on the stack as
 * STACK_ZERO, since the helper is first met by the verifier after
 * the "var = 0" assignment. This state will however be wrongly used
 * also for the "var = 1" case, so the verifier assumes "var" is always
 * 0 and will replace the NULL assignment with nops, because the
 * search pruning prevents it from exploring the faulty branch.
 */
bpf_ktime_get_ns();

if (var)
        *(long *)0 = 0xbeef;

Fix the issue by making sure that the stack is fully explored before
returning a positive comparison result.

Also attach a couple tests that highlight the bad behavior. In the first
test, without this fix instructions 16 and 17 are replaced with nops
instead of being rejected by the verifier.

The second test, instead, allows a program to make a potentially illegal
read from the stack.

Fixes: cc2b14d51053 ("bpf: teach verifier to recognize zero initialized stack")
Signed-off-by: Gianluca Borello <g.borello@gmail.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 4ae46b2cba8802cbd5e4cadc34de8bbdddc49cf8..82ae580440b873432cf973f1d4c844b3769f4a2d 100644 (file)
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -4107,7 +4107,7 @@ static bool stacksafe(struct bpf_func_state *old,
 
                if (!(old->stack[spi].spilled_ptr.live & REG_LIVE_READ))
                        /* explored state didn't use this */
-                       return true;
+                       continue;
 
                if (old->stack[spi].slot_type[i % BPF_REG_SIZE] == STACK_INVALID)
                        continue;

diff --git a/tools/testing/selftests/bpf/test_verifier.c b/tools/testing/selftests/bpf/test_verifier.c
index 3bacff0d6f917c844ec34ac1a1e80cd113a9d199..5e79515d10c5ddcee6658f2b5b4a7cff41fca7f0 100644 (file)
--- a/tools/testing/selftests/bpf/test_verifier.c
+++ b/tools/testing/selftests/bpf/test_verifier.c
@@ -9715,6 +9715,57 @@ static struct bpf_test tests[] = {
                .result = REJECT,
                .prog_type = BPF_PROG_TYPE_XDP,
        },
+       {
+               "search pruning: all branches should be verified (nop operation)",
+               .insns = {
+                       BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
+                       BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
+                       BPF_ST_MEM(BPF_DW, BPF_REG_2, 0, 0),
+                       BPF_LD_MAP_FD(BPF_REG_1, 0),
+                       BPF_EMIT_CALL(BPF_FUNC_map_lookup_elem),
+                       BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 11),
+                       BPF_LDX_MEM(BPF_DW, BPF_REG_3, BPF_REG_0, 0),
+                       BPF_JMP_IMM(BPF_JEQ, BPF_REG_3, 0xbeef, 2),
+                       BPF_MOV64_IMM(BPF_REG_4, 0),
+                       BPF_JMP_A(1),
+                       BPF_MOV64_IMM(BPF_REG_4, 1),
+                       BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_4, -16),
+                       BPF_EMIT_CALL(BPF_FUNC_ktime_get_ns),
+                       BPF_LDX_MEM(BPF_DW, BPF_REG_5, BPF_REG_10, -16),
+                       BPF_JMP_IMM(BPF_JEQ, BPF_REG_5, 0, 2),
+                       BPF_MOV64_IMM(BPF_REG_6, 0),
+                       BPF_ST_MEM(BPF_DW, BPF_REG_6, 0, 0xdead),
+                       BPF_EXIT_INSN(),
+               },
+               .fixup_map1 = { 3 },
+               .errstr = "R6 invalid mem access 'inv'",
+               .result = REJECT,
+               .prog_type = BPF_PROG_TYPE_TRACEPOINT,
+       },
+       {
+               "search pruning: all branches should be verified (invalid stack access)",
+               .insns = {
+                       BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
+                       BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
+                       BPF_ST_MEM(BPF_DW, BPF_REG_2, 0, 0),
+                       BPF_LD_MAP_FD(BPF_REG_1, 0),
+                       BPF_EMIT_CALL(BPF_FUNC_map_lookup_elem),
+                       BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 8),
+                       BPF_LDX_MEM(BPF_DW, BPF_REG_3, BPF_REG_0, 0),
+                       BPF_MOV64_IMM(BPF_REG_4, 0),
+                       BPF_JMP_IMM(BPF_JEQ, BPF_REG_3, 0xbeef, 2),
+                       BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_4, -16),
+                       BPF_JMP_A(1),
+                       BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_4, -24),
+                       BPF_EMIT_CALL(BPF_FUNC_ktime_get_ns),
+                       BPF_LDX_MEM(BPF_DW, BPF_REG_5, BPF_REG_10, -16),
+                       BPF_EXIT_INSN(),
+               },
+               .fixup_map1 = { 3 },
+               .errstr = "invalid read from stack off -16+0 size 8",
+               .result = REJECT,
+               .prog_type = BPF_PROG_TYPE_TRACEPOINT,
+       },
 };
 
 static int probe_filter_length(const struct bpf_insn *fp)