[PATCH v3] tools/mm: Fix slabinfo crash when MAX_SLABS is exceeded

[PATCH v3] tools/mm: Fix slabinfo crash when MAX_SLABS is exceeded

[Marc Dionne]

From: Marc Dionne <marc.dionne@auristor.com>

The number of slabs can easily exceed the hard coded MAX_SLABS in the
slabinfo tool, causing it to overwrite memory and crash.

Increase the value of MAX_SLABS, and check if that has been exceeded for
each new slab, instead of at the end when it's already too late.  Also
move the check for MAX_ALIASES into the loop body.

Signed-off-by: Marc Dionne <marc.dionne@auristor.com>
---
 tools/mm/slabinfo.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/tools/mm/slabinfo.c b/tools/mm/slabinfo.c
index cfaeaea71042..bf770101929a 100644
--- a/tools/mm/slabinfo.c
+++ b/tools/mm/slabinfo.c
@@ -21,7 +21,7 @@
 #include <regex.h>
 #include <errno.h>
 
-#define MAX_SLABS 500
+#define MAX_SLABS 2000
 #define MAX_ALIASES 500
 #define MAX_NODES 1024
 
@@ -1240,6 +1240,8 @@ static void read_slab_dir(void)
 				p--;
 			alias->ref = strdup(p);
 			alias++;
+			if (alias - aliasinfo == MAX_ALIASES)
+				fatal("Too many aliases\n");
 			break;
 		   case DT_DIR:
 			if (chdir(de->d_name))
@@ -1301,6 +1303,8 @@ static void read_slab_dir(void)
 			if (slab->name[0] == ':')
 				alias_targets++;
 			slab++;
+			if (slab - slabinfo == MAX_SLABS)
+				fatal("Too many slabs\n");
 			break;
 		   default :
 			fatal("Unknown file type %lx\n", de->d_type);
@@ -1310,10 +1314,6 @@ static void read_slab_dir(void)
 	slabs = slab - slabinfo;
 	actual_slabs = slabs;
 	aliases = alias - aliasinfo;
-	if (slabs > MAX_SLABS)
-		fatal("Too many slabs\n");
-	if (aliases > MAX_ALIASES)
-		fatal("Too many aliases\n");
 }
 
 static void output_slabs(void)
-- 
2.47.0

Re: [PATCH v3] tools/mm: Fix slabinfo crash when MAX_SLABS is exceeded

[Andrew Morton]

On Wed, 30 Oct 2024 10:32:08 -0300 Marc Dionne <marc.c.dionne@gmail.com> wrote:

> From: Marc Dionne <marc.dionne@auristor.com>
> 
> The number of slabs can easily exceed the hard coded MAX_SLABS in the
> slabinfo tool, causing it to overwrite memory and crash.
> 
> Increase the value of MAX_SLABS, and check if that has been exceeded for
> each new slab, instead of at the end when it's already too late.  Also
> move the check for MAX_ALIASES into the loop body.
> 
> @@ -1240,6 +1240,8 @@ static void read_slab_dir(void)
>  				p--;
>  			alias->ref = strdup(p);
>  			alias++;
> +			if (alias - aliasinfo == MAX_ALIASES)
> +				fatal("Too many aliases\n");

Again, this is not correct.  It has a potential off-by-one error.

If at this point, (alias - aliasinfo == MAX_ALIASES), we *do not know*
whether there are "Too many aliases".  Because the parsing might have
reached the end of input, in which case we're fine.

A fix for this is to check for an invalid `alias' immediately before we
use it, as I described in the previous email.