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Message-Id: <20230330081220.1128115-1-matthias.goergens@gmail.com>
Date: Thu, 30 Mar 2023 16:12:20 +0800
From: Matthias Goergens <matthias.goergens@...il.com>
To: musl@...ts.openwall.com
Cc: Matthias Goergens <matthias.goergens@...il.com>
Subject: [PATCH] Fix UB in getmntent_r on extremely long lines
8974ef2124118e4ed8cad7ee0534b36e5c584c4e tried to fix mishandling of
extremely long lines.
Here's the relevant code snippet:
```
len = strlen(linebuf);
if (len > INT_MAX) continue;
for (i = 0; i < sizeof n / sizeof *n; i++) n[i] = len;
sscanf(linebuf, " %n%*s%n %n%*s%n %n%*s%n %n%*s%n %d %d",
n, n+1, n+2, n+3, n+4, n+5, n+6, n+7,
&mnt->mnt_freq, &mnt->mnt_passno);
} while (linebuf[n[0]] == '#' || n[1]==len);
```
Alas, that introduced undefined behaviour: if the very first line
handled in the function is extremely long, `n` stays uninitialised, and
thus accessing `n[0]` and `n[1]` is UB.
If we handle a few sane lines before hitting a crazy long line, we don't
hit C-level undefined behaviour, but the function arguably still does
the wrong thing.
The man page says:
> The getmntent() and getmntent_r() functions return a pointer to the
> mntent structure or NULL on failure.
So this patch does exactly that: return NULL to inform the caller that
an error occured.
---
src/misc/mntent.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/src/misc/mntent.c b/src/misc/mntent.c
index d404fbe3..d91c4964 100644
--- a/src/misc/mntent.c
+++ b/src/misc/mntent.c
@@ -43,7 +43,7 @@ struct mntent *getmntent_r(FILE *f, struct mntent *mnt, char *linebuf, int bufle
}
len = strlen(linebuf);
- if (len > INT_MAX) continue;
+ if (len > INT_MAX) return NULL;
for (i = 0; i < sizeof n / sizeof *n; i++) n[i] = len;
sscanf(linebuf, " %n%*s%n %n%*s%n %n%*s%n %n%*s%n %d %d",
n, n+1, n+2, n+3, n+4, n+5, n+6, n+7,
--
2.40.0
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