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dc84dbbcc9
Commit a657182a5c ("bpf: Don't use tnum_range on array range checking
for poke descriptors") has shown that using tnum_range() as argument to
tnum_in() can lead to misleading code that looks like tight bound check
when in fact the actual allowed range is much wider.
Document such behavior to warn against its usage in general, and suggest
some scenario where result can be trusted.
Signed-off-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/984b37f9fdf7ac36831d2137415a4a915744c1b6.1661462653.git.daniel@iogearbox.net
Link: https://www.openwall.com/lists/oss-security/2022/08/26/1
Link: https://lore.kernel.org/bpf/20220831031907.16133-3-shung-hsi.yu@suse.com
Link: https://lore.kernel.org/bpf/20220831031907.16133-2-shung-hsi.yu@suse.com
117 lines
4 KiB
C
117 lines
4 KiB
C
/* tnum: tracked (or tristate) numbers
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*
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* A tnum tracks knowledge about the bits of a value. Each bit can be either
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* known (0 or 1), or unknown (x). Arithmetic operations on tnums will
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* propagate the unknown bits such that the tnum result represents all the
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* possible results for possible values of the operands.
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*/
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#ifndef _LINUX_TNUM_H
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#define _LINUX_TNUM_H
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#include <linux/types.h>
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struct tnum {
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u64 value;
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u64 mask;
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};
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/* Constructors */
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/* Represent a known constant as a tnum. */
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struct tnum tnum_const(u64 value);
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/* A completely unknown value */
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extern const struct tnum tnum_unknown;
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/* An unknown value that is a superset of @min <= value <= @max.
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*
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* Could include values outside the range of [@min, @max].
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* For example tnum_range(0, 2) is represented by {0, 1, 2, *3*},
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* rather than the intended set of {0, 1, 2}.
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*/
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struct tnum tnum_range(u64 min, u64 max);
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/* Arithmetic and logical ops */
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/* Shift a tnum left (by a fixed shift) */
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struct tnum tnum_lshift(struct tnum a, u8 shift);
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/* Shift (rsh) a tnum right (by a fixed shift) */
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struct tnum tnum_rshift(struct tnum a, u8 shift);
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/* Shift (arsh) a tnum right (by a fixed min_shift) */
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struct tnum tnum_arshift(struct tnum a, u8 min_shift, u8 insn_bitness);
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/* Add two tnums, return @a + @b */
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struct tnum tnum_add(struct tnum a, struct tnum b);
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/* Subtract two tnums, return @a - @b */
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struct tnum tnum_sub(struct tnum a, struct tnum b);
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/* Bitwise-AND, return @a & @b */
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struct tnum tnum_and(struct tnum a, struct tnum b);
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/* Bitwise-OR, return @a | @b */
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struct tnum tnum_or(struct tnum a, struct tnum b);
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/* Bitwise-XOR, return @a ^ @b */
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struct tnum tnum_xor(struct tnum a, struct tnum b);
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/* Multiply two tnums, return @a * @b */
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struct tnum tnum_mul(struct tnum a, struct tnum b);
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/* Return a tnum representing numbers satisfying both @a and @b */
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struct tnum tnum_intersect(struct tnum a, struct tnum b);
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/* Return @a with all but the lowest @size bytes cleared */
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struct tnum tnum_cast(struct tnum a, u8 size);
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/* Returns true if @a is a known constant */
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static inline bool tnum_is_const(struct tnum a)
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{
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return !a.mask;
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}
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/* Returns true if @a == tnum_const(@b) */
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static inline bool tnum_equals_const(struct tnum a, u64 b)
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{
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return tnum_is_const(a) && a.value == b;
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}
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/* Returns true if @a is completely unknown */
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static inline bool tnum_is_unknown(struct tnum a)
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{
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return !~a.mask;
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}
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/* Returns true if @a is known to be a multiple of @size.
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* @size must be a power of two.
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*/
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bool tnum_is_aligned(struct tnum a, u64 size);
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/* Returns true if @b represents a subset of @a.
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*
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* Note that using tnum_range() as @a requires extra cautions as tnum_in() may
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* return true unexpectedly due to tnum limited ability to represent tight
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* range, e.g.
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*
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* tnum_in(tnum_range(0, 2), tnum_const(3)) == true
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*
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* As a rule of thumb, if @a is explicitly coded rather than coming from
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* reg->var_off, it should be in form of tnum_const(), tnum_range(0, 2**n - 1),
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* or tnum_range(2**n, 2**(n+1) - 1).
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*/
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bool tnum_in(struct tnum a, struct tnum b);
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/* Formatting functions. These have snprintf-like semantics: they will write
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* up to @size bytes (including the terminating NUL byte), and return the number
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* of bytes (excluding the terminating NUL) which would have been written had
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* sufficient space been available. (Thus tnum_sbin always returns 64.)
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*/
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/* Format a tnum as a pair of hex numbers (value; mask) */
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int tnum_strn(char *str, size_t size, struct tnum a);
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/* Format a tnum as tristate binary expansion */
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int tnum_sbin(char *str, size_t size, struct tnum a);
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/* Returns the 32-bit subreg */
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struct tnum tnum_subreg(struct tnum a);
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/* Returns the tnum with the lower 32-bit subreg cleared */
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struct tnum tnum_clear_subreg(struct tnum a);
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/* Returns the tnum with the lower 32-bit subreg set to value */
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struct tnum tnum_const_subreg(struct tnum a, u32 value);
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/* Returns true if 32-bit subreg @a is a known constant*/
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static inline bool tnum_subreg_is_const(struct tnum a)
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{
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return !(tnum_subreg(a)).mask;
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}
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#endif /* _LINUX_TNUM_H */
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