commit 4812441d3465f4f2fc6763ee644f79d5a9c8661b Author: Nick Mathewson nickm@torproject.org Date: Fri Apr 7 10:47:16 2017 -0400

Never read off the end of a buffer in base32_encode()

When we "fixed" #18280 in 4e4a7d2b0c199227252a742541461ec4cc35d358 in 0291 it appears that we introduced a bug: The base32_encode function can read off the end of the input buffer, if the input buffer size modulo 5 is not equal to 0 or 3.

This is not completely horrible, for two reasons: * The extra bits that are read are never actually used: so this is only a crash when asan is enabled, in the worst case. Not a data leak.

* The input sizes passed to base32_encode are only ever multiples of 5. They are all either DIGEST_LEN (20), REND_SERVICE_ID_LEN (10), sizeof(rand_bytes) in addressmap.c (10), or an input in crypto.c that is forced to a multiple of 5.

So this bug can't actually trigger in today's Tor.

Closes bug 21894; bugfix on 0.2.9.1-alpha. --- changes/bug21894_029 | 5 +++++ src/common/util_format.c | 7 ++++--- 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/changes/bug21894_029 b/changes/bug21894_029 new file mode 100644 index 0000000..e3a84fa --- /dev/null +++ b/changes/bug21894_029 @@ -0,0 +1,5 @@ + o Minor bugfixes (crash prevention): + - Fix an (currently untriggerable, but potentially dangerous) crash + bug when base32-encoding inputs whose sizes are not a multiple of + 5. Fixes bug 21894; bugfix on 0.2.9.1-alpha. + diff --git a/src/common/util_format.c b/src/common/util_format.c index aef9db8..a460842 100644 --- a/src/common/util_format.c +++ b/src/common/util_format.c @@ -51,9 +51,10 @@ base32_encode(char *dest, size_t destlen, const char *src, size_t srclen)

for (i=0,bit=0; bit < nbits; ++i, bit+=5) { /* set v to the 16-bit value starting at src[bits/8], 0-padded. */ - v = ((uint8_t)src[bit/8]) << 8; - if (bit+5<nbits) - v += (uint8_t)src[(bit/8)+1]; + size_t idx = bit / 8; + v = ((uint8_t)src[idx]) << 8; + if (idx+1 < srclen) + v += (uint8_t)src[idx+1]; /* set u to the 5-bit value at the bit'th bit of buf. */ u = (v >> (11-(bit%8))) & 0x1F; dest[i] = BASE32_CHARS[u];