Re: [PATCH v2] tools/mm: Fix slabinfo crash when MAX_SLABS is exceeded

[Andrew Morton]

On Tue, 29 Oct 2024 13:13:41 -0300 Marc Dionne <marc.c.dionne@xxxxxxxxx> wrote:

> From: Marc Dionne <marc.dionne@xxxxxxxxxxxx>
> 
> 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.

Thanks.

> --- 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 1000

That isn't a very large increase.

>  #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;

This could be improved - if the number of slabs is exactly equal to
MAX_SLABS we'll unnecessarily report an error.  Wouldn't this
alteration be better?  

--- a/tools/mm/slabinfo.c~tools-mm-fix-slabinfo-crash-when-max_slabs-is-exceeded-fix
+++ a/tools/mm/slabinfo.c
@@ -1228,6 +1228,8 @@ static void read_slab_dir(void)
 				continue;
 		switch (de->d_type) {
 		   case DT_LNK:
+			if (alias - aliasinfo >= MAX_ALIASES)
+				fatal("Too many aliases\n");
 			alias->name = strdup(de->d_name);
 			count = readlink(de->d_name, buffer, sizeof(buffer)-1);
 
@@ -1240,10 +1242,10 @@ 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 (slab - slabinfo >= MAX_SLABS)
+				fatal("Too many slabs\n");
 			if (chdir(de->d_name))
 				fatal("Unable to access slab %s\n", slab->name);
 			slab->name = strdup(de->d_name);
@@ -1305,8 +1307,6 @@ 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);
_