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util/nvmutil: fix entropy issue

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the time difference used here could go negative, which
would overflow in the xor op on mix, leading to a biased
entropy pool. we want to ensure that they numbers do
not overflow, because here they are cast to unsigned
which would then produce very large numbers.

Signed-off-by: Leah Rowe <leah@libreboot.org>
This commit is contained in:
Leah Rowe
2026-03-13 14:10:10 +00:00
parent 475c50932e
commit 0c9bcaaba8
1 changed files with 10 additions and 1 deletions
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util/nvmutil/nvmutil.c
+10 -1
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@@ -1172,6 +1172,7 @@ entropy_jitter(void)
{ {
struct timeval a, b; struct timeval a, b;
unsigned long mix = 0; unsigned long mix = 0;
long mix_diff;
int i; int i;
for (i = 0; i < 8; i++) { for (i = 0; i < 8; i++) {
@@ -1179,7 +1180,15 @@ entropy_jitter(void)
getpid(); getpid();
gettimeofday(&b, NULL); gettimeofday(&b, NULL);
mix ^= (unsigned long)(b.tv_usec - a.tv_usec); /*
* prevent negative numbers to prevent overflow,
* which would bias rand to large numbers
*/
mix_diff = (long)(b.tv_usec - a.tv_usec);
if (mix_diff < 0)
mix_diff = -mix_diff;
mix ^= (unsigned long)(mix_diff);
mix ^= (unsigned long)&mix; mix ^= (unsigned long)&mix;
} }