Re: [PATCH] tools/xenstored: Prevent a buffer overflow in dump_state_node_perms()

[Luca Fancellu <luca.fancellu@xxxxxxx>]

> On 6 May 2021, at 17:12, Julien Grall <julien@xxxxxxx> wrote:
> 
> From: Julien Grall <jgrall@xxxxxxxxxx>
> 
> ASAN reported one issue when Live Updating Xenstored:
> 
> =================================================================
> ==873==ERROR: AddressSanitizer: stack-buffer-overflow on address 
> 0x7ffc194f53e0 at pc 0x555c6b323292 bp 0x7ffc194f5340 sp 0x7ffc194f5338
> WRITE of size 1 at 0x7ffc194f53e0 thread T0
>    #0 0x555c6b323291 in dump_state_node_perms 
> xen/tools/xenstore/xenstored_core.c:2468
>    #1 0x555c6b32746e in dump_state_special_node 
> xen/tools/xenstore/xenstored_domain.c:1257
>    #2 0x555c6b32a702 in dump_state_special_nodes 
> xen/tools/xenstore/xenstored_domain.c:1273
>    #3 0x555c6b32ddb3 in lu_dump_state 
> xen/tools/xenstore/xenstored_control.c:521
>    #4 0x555c6b32e380 in do_lu_start xen/tools/xenstore/xenstored_control.c:660
>    #5 0x555c6b31b461 in call_delayed xen/tools/xenstore/xenstored_core.c:278
>    #6 0x555c6b32275e in main xen/tools/xenstore/xenstored_core.c:2357
>    #7 0x7f95eecf3d09 in __libc_start_main ../csu/libc-start.c:308
>    #8 0x555c6b3197e9 in _start (/usr/local/sbin/xenstored+0xc7e9)
> 
> Address 0x7ffc194f53e0 is located in stack of thread T0 at offset 80 in frame
>    #0 0x555c6b32713e in dump_state_special_node 
> xen/tools/xenstore/xenstored_domain.c:1232
> 
>  This frame has 2 object(s):
>    [32, 40) 'head' (line 1233)
>    [64, 80) 'sn' (line 1234) <== Memory access at offset 80 overflows this 
> variable
> 
> This is happening because the callers are passing a pointer to a variable
> allocated on the stack. However, the field perms is a dynamic array, so
> Xenstored will end up to read outside of the variable.
> 
> Rework the code so the permissions are written one by one in the fd.
> 
> Fixes: ed6eebf17d2c ("tools/xenstore: dump the xenstore state for live 
> update")
> Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
> ---
> tools/xenstore/xenstored_core.c   | 26 ++++++++++++++------------
> tools/xenstore/xenstored_core.h   |  3 +--
> tools/xenstore/xenstored_domain.c |  2 +-
> 3 files changed, 16 insertions(+), 15 deletions(-)
> 
> diff --git a/tools/xenstore/xenstored_core.c b/tools/xenstore/xenstored_core.c
> index d54a6042a9f7..f68da12b5b23 100644
> --- a/tools/xenstore/xenstored_core.c
> +++ b/tools/xenstore/xenstored_core.c
> @@ -2447,34 +2447,36 @@ const char *dump_state_buffered_data(FILE *fp, const 
> struct connection *c,
>       return NULL;
> }
> 
> -const char *dump_state_node_perms(FILE *fp, struct xs_state_node *sn,
> -                               const struct xs_permissions *perms,
> +const char *dump_state_node_perms(FILE *fp, const struct xs_permissions 
> *perms,
>                                 unsigned int n_perms)
> {
>       unsigned int p;
> 
>       for (p = 0; p < n_perms; p++) {
> +             struct xs_state_node_perm sp;
> +
>               switch ((int)perms[p].perms & ~XS_PERM_IGNORE) {
>               case XS_PERM_READ:
> -                     sn->perms[p].access = XS_STATE_NODE_PERM_READ;
> +                     sp.access = XS_STATE_NODE_PERM_READ;
>                       break;
>               case XS_PERM_WRITE:
> -                     sn->perms[p].access = XS_STATE_NODE_PERM_WRITE;
> +                     sp.access = XS_STATE_NODE_PERM_WRITE;
>                       break;
>               case XS_PERM_READ | XS_PERM_WRITE:
> -                     sn->perms[p].access = XS_STATE_NODE_PERM_BOTH;
> +                     sp.access = XS_STATE_NODE_PERM_BOTH;
>                       break;
>               default:
> -                     sn->perms[p].access = XS_STATE_NODE_PERM_NONE;
> +                     sp.access = XS_STATE_NODE_PERM_NONE;
>                       break;
>               }
> -             sn->perms[p].flags = (perms[p].perms & XS_PERM_IGNORE)
> +             sp.flags = (perms[p].perms & XS_PERM_IGNORE)
>                                    ? XS_STATE_NODE_PERM_IGNORE : 0;
> -             sn->perms[p].domid = perms[p].id;
> -     }
> +             sp.domid = perms[p].id;
> 
> -     if (fwrite(sn->perms, sizeof(*sn->perms), n_perms, fp) != n_perms)
> -             return "Dump node permissions error";
> +             if (fwrite(&sp, sizeof(sp), 1, fp) != 1)
> +                     return "Dump node permission error";
> +
> +     }
> 
>       return NULL;
> }
> @@ -2519,7 +2521,7 @@ static const char *dump_state_node_tree(FILE *fp, char 
> *path)
>       if (fwrite(&sn, sizeof(sn), 1, fp) != 1)
>               return "Dump node state error";
> 
> -     ret = dump_state_node_perms(fp, &sn, hdr->perms, hdr->num_perms);
> +     ret = dump_state_node_perms(fp, hdr->perms, hdr->num_perms);
>       if (ret)
>               return ret;
> 
> diff --git a/tools/xenstore/xenstored_core.h b/tools/xenstore/xenstored_core.h
> index 1cdbc3dcb5f7..b50ea3f57d5a 100644
> --- a/tools/xenstore/xenstored_core.h
> +++ b/tools/xenstore/xenstored_core.h
> @@ -271,8 +271,7 @@ const char *dump_state_buffered_data(FILE *fp, const 
> struct connection *c,
>                                    const struct connection *conn,
>                                    struct xs_state_connection *sc);
> const char *dump_state_nodes(FILE *fp, const void *ctx);
> -const char *dump_state_node_perms(FILE *fp, struct xs_state_node *sn,
> -                               const struct xs_permissions *perms,
> +const char *dump_state_node_perms(FILE *fp, const struct xs_permissions 
> *perms,
>                                 unsigned int n_perms);
> 
> void read_state_global(const void *ctx, const void *state);
> diff --git a/tools/xenstore/xenstored_domain.c 
> b/tools/xenstore/xenstored_domain.c
> index 3d4d0649a243..580ed454a3f5 100644
> --- a/tools/xenstore/xenstored_domain.c
> +++ b/tools/xenstore/xenstored_domain.c
> @@ -1254,7 +1254,7 @@ static const char *dump_state_special_node(FILE *fp, 
> const char *name,
>       if (fwrite(&sn, sizeof(sn), 1, fp) != 1)
>               return "Dump special node error";
> 
> -     ret = dump_state_node_perms(fp, &sn, perms->p, perms->num);
> +     ret = dump_state_node_perms(fp, perms->p, perms->num);
>       if (ret)
>               return ret;
> 
> -- 
> 2.17.1
> 
> 

Tested on FVP and another arm board, basic testing (run Xen, dom0, run one/two 
guests) - Everything fine.

Reviewed-by: Luca Fancellu <luca.fancellu@xxxxxxx>

Cheers,
Luca