[PATCH 2/2] wrapper: use a CSPRNG to generate random file names
- From
brian m. carlson <sandals@crustytoothpaste.net>
- Date
- Nov 16, 2021, 03:35 UTC
- Message-ID
- <20211116033542.3247094-3-sandals@crustytoothpaste.net>
- In-Reply-To
- <20211116033542.3247094-1-sandals@crustytoothpaste.net>
The current way we generate random file names is by taking the seconds and microseconds, plus the PID, and mixing them together, then encoding them. If this fails, we increment the value by 7777, and try again up to TMP_MAX times.
Unfortunately, this is not the best idea from a security perspective. If we're writing into TMPDIR, an attacker can guess these values easily and prevent us from creating any temporary files at all by creating them all first. POSIX only requires TMP_MAX to be 25, so this is achievable in some contexts, even if unlikely to occur in practice.
Fortunately, we can simply solve this by using the system cryptographically secure pseudorandom number generator (CSPRNG) to generate a random 64-bit value, and use that as before. Note that there is still a small bias here, but because a six-character sequence chosen out of 62 characters provides about 36 bits of entropy, the bias here is less than 2^-28, which is acceptable, especially considering we'll retry several times.
Note that the use of a CSPRNG in generating temporary file names is also used in many libcs. glibc recently changed from an approach similar to ours to using a CSPRNG, and FreeBSD and OpenBSD also use a CSPRNG in this case. Even if the likelihood of an attack is low, we should still be at least as responsible in creating temporary files as libc is.
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net> --- wrapper.c | 15 ++++----------- 1 file changed, 4 insertions(+), 11 deletions(-)
diff --git a/wrapper.c b/wrapper.c index 0046f32e46..0cdb5b18ff 100644 --- a/wrapper.c +++ b/wrapper.c @@ -463,8 +463,6 @@ int git_mkstemps_mode(char *pattern, int suffix_len, int mode) static const int num_letters = ARRAY_SIZE(letters) - 1; static const char x_pattern[] = "XXXXXX"; static const int num_x = ARRAY_SIZE(x_pattern) - 1; - uint64_t value; - struct timeval tv; char *filename_template; size_t len; int fd, count; @@ -485,12 +483,13 @@ int git_mkstemps_mode(char *pattern, int suffix_len, int mode) * Replace pattern's XXXXXX characters with randomness. * Try TMP_MAX different filenames. */ - gettimeofday(&tv, NULL); - value = ((uint64_t)tv.tv_usec << 16) ^ tv.tv_sec ^ getpid(); filename_template = &pattern[len - num_x - suffix_len]; for (count = 0; count < TMP_MAX; ++count) { - uint64_t v = value; int i; + uint64_t v; + if (csprng_bytes(&v, sizeof(v)) < 0) + return -1; + /* Fill in the random bits. */ for (i = 0; i < num_x; i++) { filename_template[i] = letters[v % num_letters]; @@ -506,12 +505,6 @@ int git_mkstemps_mode(char *pattern, int suffix_len, int mode) */ if (errno != EEXIST) break; - /* - * This is a random value. It is only necessary that - * the next TMP_MAX values generated by adding 7777 to - * VALUE are different with (module 2^32). - */ - value += 7777; } /* We return the null string if we can't find a unique file name. */ pattern[0] = '\0';