bpf/verifier: track signed and unsigned min/max values

Allows us to, sometimes, combine information from a signed check of one
 bound and an unsigned check of the other.
We now track the full range of possible values, rather than restricting
 ourselves to [0, 1<<30) and considering anything beyond that as
 unknown.  While this is probably not necessary, it makes the code more
 straightforward and symmetrical between signed and unsigned bounds.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Edward Cree 2017-08-07 15:26:36 +01:00 committed by David S. Miller
parent f1174f77b5
commit b03c9f9fdc
4 changed files with 461 additions and 317 deletions

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@ -11,11 +11,15 @@
#include <linux/filter.h> /* for MAX_BPF_STACK */
#include <linux/tnum.h>
/* Just some arbitrary values so we can safely do math without overflowing and
* are obviously wrong for any sort of memory access.
*/
#define BPF_REGISTER_MAX_RANGE (1024 * 1024 * 1024)
#define BPF_REGISTER_MIN_RANGE -1
/* Maximum variable offset umax_value permitted when resolving memory accesses.
* In practice this is far bigger than any realistic pointer offset; this limit
* ensures that umax_value + (int)off + (int)size cannot overflow a u64.
*/
#define BPF_MAX_VAR_OFF (1ULL << 31)
/* Maximum variable size permitted for ARG_CONST_SIZE[_OR_ZERO]. This ensures
* that converting umax_value to int cannot overflow.
*/
#define BPF_MAX_VAR_SIZ INT_MAX
struct bpf_reg_state {
enum bpf_reg_type type;
@ -36,7 +40,7 @@ struct bpf_reg_state {
* came from, when one is tested for != NULL.
*/
u32 id;
/* These three fields must be last. See states_equal() */
/* These five fields must be last. See states_equal() */
/* For scalar types (SCALAR_VALUE), this represents our knowledge of
* the actual value.
* For pointer types, this represents the variable part of the offset
@ -49,9 +53,10 @@ struct bpf_reg_state {
* These refer to the same value as var_off, not necessarily the actual
* contents of the register.
*/
s64 min_value;
u64 max_value;
bool value_from_signed;
s64 smin_value; /* minimum possible (s64)value */
s64 smax_value; /* maximum possible (s64)value */
u64 umin_value; /* minimum possible (u64)value */
u64 umax_value; /* maximum possible (u64)value */
};
enum bpf_stack_slot_type {

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@ -17,6 +17,8 @@ struct tnum {
struct tnum tnum_const(u64 value);
/* A completely unknown value */
extern const struct tnum tnum_unknown;
/* A value that's unknown except that @min <= value <= @max */
struct tnum tnum_range(u64 min, u64 max);
/* Arithmetic and logical ops */
/* Shift a tnum left (by a fixed shift) */

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@ -17,6 +17,22 @@ struct tnum tnum_const(u64 value)
return TNUM(value, 0);
}
struct tnum tnum_range(u64 min, u64 max)
{
u64 chi = min ^ max, delta;
u8 bits = fls64(chi);
/* special case, needed because 1ULL << 64 is undefined */
if (bits > 63)
return tnum_unknown;
/* e.g. if chi = 4, bits = 3, delta = (1<<3) - 1 = 7.
* if chi = 0, bits = 0, delta = (1<<0) - 1 = 0, so we return
* constant min (since min == max).
*/
delta = (1ULL << bits) - 1;
return TNUM(min & ~delta, delta);
}
struct tnum tnum_lshift(struct tnum a, u8 shift)
{
return TNUM(a.value << shift, a.mask << shift);

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