RepoMirrors/musl
1
0
Fork 0
mirror of git://git.musl-libc.org/musl synced 2024-12-16 19:55:38 +00:00
Code Issues Packages Projects Releases Wiki Activity

fix pointer overflow bug in floating point printf

Browse Source 
large precision values could cause out-of-bounds pointer arithmetic in
computing the precision cutoff (used to avoid expensive long-precision
arithmetic when the result will be discarded). per the C standard,
this is undefined behavior. one would expect that it works anyway, and
in fact it did in most real-world cases, but it was randomly
(depending on aslr) crashing in i386 binaries running on x86_64
kernels. this is because linux puts the userspace stack near 4GB
(instead of near 3GB) when the kernel is 64-bit, leading to the
out-of-bounds pointer arithmetic overflowing past the end of address
space and giving a very low pointer value, which then compared lower
than a pointer it should have been higher than.

the new code rearranges the arithmetic so that no overflow can occur.

while this bug could crash printf with memory corruption, it's
unlikely to have security impact in real-world applications since the
ability to provide an extremely large field precision value under
attacker-control is required to trigger the bug.
This commit is contained in:
Rich Felker 2012-06-19 21:41:43 -04:00
parent 25c8444919
commit 914949d321
1 changed files with 3 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

6
src/stdio/vfprintf.c
View File

@ -296,7 +296,7 @@ static int fmt_fp(FILE *f, long double y, int w, int p, int fl, int t)
e2-=sh;
}
while (e2<0) {
uint32_t carry=0, *z2;
uint32_t carry=0, *b;
int sh=MIN(9,-e2);
for (d=a; d<z; d++) {
uint32_t rm = *d & (1<<sh)-1;
@ -306,8 +306,8 @@ static int fmt_fp(FILE *f, long double y, int w, int p, int fl, int t)
if (!*a) a++;
if (carry) *z++ = carry;
/* Avoid (slow!) computation past requested precision */
z2 = ((t|32)=='f' ? r : a) + 2 + p/9;
z = MIN(z, z2);
b = (t|32)=='f' ? r : a;
if (z-b > 2+p/9) z = b+2+p/9;
e2+=sh;
}