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authorRich Felker <dalias@aerifal.cx>2022-06-03 18:54:41 -0400
committerRich Felker <dalias@aerifal.cx>2022-06-03 18:54:41 -0400
commit4100279825c17807bdabf1c128ba4e49a1dea406 (patch)
tree5b9477609bce9bb38843ac5b38d31a59244335d6
parent6c858d6fd4df8b5498ef2cae66c8f3c3eff1587b (diff)
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remove random filename obfuscation that leaks ASLR informationHEADmaster
the __randname function is used by various temp file creation interfaces as a backend to produce a name to attempt using. it does not have to produce results that are safe against guessing, and only aims to avoid unintentional collisions. mixing the address of an object on the stack in a reversible manner leaked ASLR information, potentially allowing an attacker who can observe the temp files created and their creation timestamps to narrow down the possible ASLR state of the process that created them. there is no actual value in mixing these addresses in; it was just obfuscation. so don't do it. instead, mix the tid, just to avoid collisions if multiple processes/threads stampede to create temp files at the same moment. even without this measure, they should not collide unless the clock source is very low resolution, but it's a cheap improvement. if/when we have a guaranteed-available userspace csprng, it could be used here instead. even though there is no need for cryptographic entropy here, it would avoid having to reason about clock resolution and such to determine whether the behavior is nice.
-rw-r--r--src/temp/__randname.c3
1 files changed, 2 insertions, 1 deletions
diff --git a/src/temp/__randname.c b/src/temp/__randname.c
index 2bce37a0..1425badc 100644
--- a/src/temp/__randname.c
+++ b/src/temp/__randname.c
@@ -1,5 +1,6 @@
#include <time.h>
#include <stdint.h>
+#include "pthread_impl.h"
/* This assumes that a check for the
template size has already been made */
@@ -10,7 +11,7 @@ char *__randname(char *template)
unsigned long r;
__clock_gettime(CLOCK_REALTIME, &ts);
- r = ts.tv_nsec*65537 ^ (uintptr_t)&ts / 16 + (uintptr_t)template;
+ r = ts.tv_nsec + __pthread_self()->tid * 65537UL;
for (i=0; i<6; i++, r>>=5)
template[i] = 'A'+(r&15)+(r&16)*2;