expand: Fix buffer overflow in expandmeta

Commit Message

Harald van Dijk March 26, 2018, 4:37 a.m. UTC

On 3/26/18 4:54 AM, Herbert Xu wrote:
> On Mon, Mar 26, 2018 at 03:03:38AM +0200, Harald van Dijk wrote:
>>
>> This was already the case before your patch, but on systems that flat out
>> reject paths longer than PATH_MAX (that includes GNU/Linux), it seems weird
>> that expansion can produce paths which then don't work.
> 
> PATH_MAX only applies in certain cases.  Besides, you can always
> cd into the directories one level at a time to circumvent it.

In other words, when you change the path to a different one than what 
globbing produced. That was kind of my point. :)

> Besides, even if we did impose a limit here
> we'd still have to check that limit at every recursion level which
> means that the code won't be that much simpler.

Proof of concept attached. The resulting code is about 100 bytes smaller 
at -Os than the dynamic reallocation approach. It's possible to further 
reduce that with a statically allocated buffer, but of course that means 
a constant memory cost at run time.

Cheers,
Harald van Dijk

Comments

Herbert Xu March 26, 2018, 5:12 a.m. UTC | #1

On Mon, Mar 26, 2018 at 06:37:45AM +0200, Harald van Dijk wrote:
>
> >Besides, even if we did impose a limit here
> >we'd still have to check that limit at every recursion level which
> >means that the code won't be that much simpler.
> 
> Proof of concept attached. The resulting code is about 100 bytes smaller at
> -Os than the dynamic reallocation approach. It's possible to further reduce
> that with a statically allocated buffer, but of course that means a constant
> memory cost at run time.

FWIW your patch when built with -O2 with gcc 4.7.2 is actually
16 bytes bigger than my version.

> @@ -1344,6 +1335,8 @@ expmeta(char *enddir, char *name)
>  			metaflag++;
>  		p = name;
>  		do {
> +			if (enddir == expdir + PATH_MAX)
> +				return;
>  			if (*p == '\\')
>  				p++;
>  			*enddir++ = *p;

Also there is another loop in between these two hunks that also
needs to check enddir.  My version doesn't need it because the
caller will ensure that at least FILENAME bytes are available.

> @@ -1390,22 +1383,34 @@ expmeta(char *enddir, char *name)
>  		if (dp->d_name[0] == '.' && ! matchdot)
>  			continue;
>  		if (pmatch(start, dp->d_name)) {
> +			p = enddir;
> +			cp = dp->d_name;
> +			for (;;) {
> +				if (p == expdir + PATH_MAX)
> +					goto toolong;
> +				if ((*p++ = *cp++) == '\0')
> +					break;
> +			}
>  			if (atend) {
> -				scopy(dp->d_name, enddir);
>  				addfname(expdir);
>  			} else {
> -				for (p = enddir, cp = dp->d_name;
> -				     (*p++ = *cp++) != '\0';)
> -					continue;
>  				p[-1] = '/';
> -				expmeta(p, endname);
> +				expmeta1(expdir, p, endname);
>  			}
>  		}
> +toolong: ;
>  	}
>  	closedir(dirp);
>  	if (! atend)
>  		endname[-esc - 1] = esc ? '\\' : '/';
>  }

Thanks,

Harald van Dijk March 26, 2018, 6:19 a.m. UTC | #2

On 3/26/18 7:12 AM, Herbert Xu wrote:
> FWIW your patch when built with -O2 with gcc 4.7.2 is actually
> 16 bytes bigger than my version.

Interesting. Not sure what might cause that.

>> @@ -1344,6 +1335,8 @@ expmeta(char *enddir, char *name)
>>   			metaflag++;
>>   		p = name;
>>   		do {
>> +			if (enddir == expdir + PATH_MAX)
>> +				return;
>>   			if (*p == '\\')
>>   				p++;
>>   			*enddir++ = *p;
> 
> Also there is another loop in between these two hunks that also
> needs to check enddir.

Indeed, thanks.

Cheers,
Harald van Dijk
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Herbert Xu March 26, 2018, 8:13 a.m. UTC | #3

On Mon, Mar 26, 2018 at 08:19:02AM +0200, Harald van Dijk wrote:
> On 3/26/18 7:12 AM, Herbert Xu wrote:
> >FWIW your patch when built with -O2 with gcc 4.7.2 is actually
> >16 bytes bigger than my version.
> 
> Interesting. Not sure what might cause that.

gcc likes to unroll things a lot which tends to duplicate code.
Sometimes I end up with three copies of the same code segment
just because gcc thinks it'll help remove branches.

None of this happens with -Os.

Cheers,

Patch

--- a/src/expand.c
+++ b/src/expand.c
@@ -122,7 +122,7 @@  STATIC void expandmeta(struct strlist *, int);
 #ifdef HAVE_GLOB
 STATIC void addglob(const glob_t *);
 #else
-STATIC void expmeta(char *, char *);
+STATIC void expmeta(char *);
 STATIC struct strlist *expsort(struct strlist *);
 STATIC struct strlist *msort(struct strlist *, int);
 #endif
@@ -1237,9 +1237,6 @@  addglob(pglob)
 
 
 #else	/* HAVE_GLOB */
-STATIC char *expdir;
-
-
 STATIC void
 expandmeta(struct strlist *str, int flag)
 {
@@ -1261,13 +1258,7 @@  expandmeta(struct strlist *str, int flag)
 
 		INTOFF;
 		p = preglob(str->text, RMESCAPE_ALLOC | RMESCAPE_HEAP);
-		{
-			int i = strlen(str->text);
-			expdir = ckmalloc(i < 2048 ? 2048 : i); /* XXX */
-		}
-
-		expmeta(expdir, p);
-		ckfree(expdir);
+		expmeta(p);
 		if (p != str->text)
 			ckfree(p);
 		INTON;
@@ -1296,7 +1287,7 @@  nometa:
  */
 
 STATIC void
-expmeta(char *enddir, char *name)
+expmeta1(char *expdir, char *enddir, char *name)
 {
 	char *p;
 	const char *cp;
@@ -1344,6 +1335,8 @@  expmeta(char *enddir, char *name)
 			metaflag++;
 		p = name;
 		do {
+			if (enddir == expdir + PATH_MAX)
+				return;
 			if (*p == '\\')
 				p++;
 			*enddir++ = *p;
@@ -1390,22 +1383,34 @@  expmeta(char *enddir, char *name)
 		if (dp->d_name[0] == '.' && ! matchdot)
 			continue;
 		if (pmatch(start, dp->d_name)) {
+			p = enddir;
+			cp = dp->d_name;
+			for (;;) {
+				if (p == expdir + PATH_MAX)
+					goto toolong;
+				if ((*p++ = *cp++) == '\0')
+					break;
+			}
 			if (atend) {
-				scopy(dp->d_name, enddir);
 				addfname(expdir);
 			} else {
-				for (p = enddir, cp = dp->d_name;
-				     (*p++ = *cp++) != '\0';)
-					continue;
 				p[-1] = '/';
-				expmeta(p, endname);
+				expmeta1(expdir, p, endname);
 			}
 		}
+toolong: ;
 	}
 	closedir(dirp);
 	if (! atend)
 		endname[-esc - 1] = esc ? '\\' : '/';
 }
+
+STATIC void
+expmeta(char *name)
+{
+	char expdir[PATH_MAX];
+	expmeta1(expdir, expdir, name);
+}
 #endif	/* HAVE_GLOB */