[PATCH 4.18 117/350] bpf/verifier: fix verifier instability

Greg Kroah-Hartman
2018-11-11T23:34:59Z

From: Alexei Starovoitov <ast@kernel.org>

[ Upstream commit a9c676bc8fc58d00eea9836fb14ee43c0346416a ]

Edward Cree says:
In check_mem_access(), for the PTR_TO_CTX case, after check_ctx_access()
has supplied a reg_type, the other members of the register state are set
appropriately. Previously reg.range was set to 0, but as it is in a
union with reg.map_ptr, which is larger, upper bytes of the latter were
left in place. This then caused the memcmp() in regsafe() to fail,
preventing some branches from being pruned (and occasionally causing the
same program to take a varying number of processed insns on repeated
verifier runs).

Fix the instability by clearing bpf_reg_state in __mark_reg_[un]known()

Fixes: f1174f77b50c ("bpf/verifier: rework value tracking")
Debugged-by: Edward Cree <ecree@solarflare.com>
Acked-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
kernel/bpf/verifier.c | 16 ++++++++--------
1 file changed, 8 insertions(+), 8 deletions(-)

--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -553,7 +553,9 @@ static void __mark_reg_not_init(struct b
*/
static void __mark_reg_known(struct bpf_reg_state *reg, u64 imm)
{
- reg->id = 0;
+ /* Clear id, off, and union(map_ptr, range) */
+ memset(((u8 *)reg) + sizeof(reg->type), 0,
+ offsetof(struct bpf_reg_state, var_off) - sizeof(reg->type));
reg->var_off = tnum_const(imm);
reg->smin_value = (s64)imm;
reg->smax_value = (s64)imm;
@@ -572,7 +574,6 @@ static void __mark_reg_known_zero(struct
static void __mark_reg_const_zero(struct bpf_reg_state *reg)
{
__mark_reg_known(reg, 0);
- reg->off = 0;
reg->type = SCALAR_VALUE;
}

@@ -683,9 +684,12 @@ static void __mark_reg_unbounded(struct
/* Mark a register as having a completely unknown (scalar) value. */
static void __mark_reg_unknown(struct bpf_reg_state *reg)
{
+ /*
+ * Clear type, id, off, and union(map_ptr, range) and
+ * padding between 'type' and union
+ */
+ memset(reg, 0, offsetof(struct bpf_reg_state, var_off));
reg->type = SCALAR_VALUE;
- reg->id = 0;
- reg->off = 0;
reg->var_off = tnum_unknown;
reg->frameno = 0;
__mark_reg_unbounded(reg);
@@ -1726,9 +1730,6 @@ static int check_mem_access(struct bpf_v
else
mark_reg_known_zero(env, regs,
value_regno);
- regs[value_regno].id = 0;
- regs[value_regno].off = 0;
- regs[value_regno].range = 0;
regs[value_regno].type = reg_type;
}

@@ -2549,7 +2550,6 @@ static int check_helper_call(struct bpf_
regs[BPF_REG_0].type = PTR_TO_MAP_VALUE_OR_NULL;
/* There is no offset yet applied, variable or fixed */
mark_reg_known_zero(env, regs, BPF_REG_0);
- regs[BPF_REG_0].off = 0;
/* remember map_ptr, so that check_map_access()
* can check 'value_size' boundary of memory access
* to map element returned from bpf_map_lookup_elem()