[libvirt] [PATCH] Avoid segfault in virt-aa-helper when handling read-only mount filesystems

John Ferlan jferlan at redhat.com
Wed Aug 24 01:13:44 UTC 2016



On 08/23/2016 08:02 PM, Rufo Dogav wrote:
> This patch fixes a segfault in virt-aa-helper caused by attempting to modify a
> string literal in situ.  It is triggered when a domain has a <filesystem> with
> type='mount' configured readonly, and libvirt is using the AppArmor security
> driver for sVirt confinement.
> ---
> 
> Thanks for the advice Martin - I completely overlooked the free.
> 
>  src/security/virt-aa-helper.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/src/security/virt-aa-helper.c b/src/security/virt-aa-helper.c
> index 49e12b9..b385d8c 100644
> --- a/src/security/virt-aa-helper.c
> +++ b/src/security/virt-aa-helper.c
> @@ -740,6 +740,7 @@ vah_add_path(virBufferPtr buf, const char *path, const char *perms, bool recursi
>      bool readonly = true;
>      bool explicit_deny_rule = true;
>      char *sub = NULL;
> +    char *perms_new = strdup(perms);

VIR_STRDUP

Then what happens if perms_new == NULL? (eg, failure to strdup).

Typically this is done via:

    if (VIR_STRDUP(perms_new, perms) < 0))
        return -1

The other problem with what you've modified is that there's a number of
places after allocation where return 0 or return rc are done, but the
memory isn't free'd...


>  
>      if (path == NULL)
>          return rc;
> @@ -764,12 +765,12 @@ vah_add_path(virBufferPtr buf, const char *path, const char *perms, bool recursi
>          return rc;
>      }
>  

Suggestion to add your VIR_STRDUP here since after here failure does the
goto cleanup.

John
> -    if (strchr(perms, 'w') != NULL) {
> +    if (strchr(perms_new, 'w') != NULL) {
>          readonly = false;
>          explicit_deny_rule = false;
>      }
>  
> -    if ((sub = strchr(perms, 'R')) != NULL) {
> +    if ((sub = strchr(perms_new, 'R')) != NULL) {
>          /* Don't write the invalid R permission, replace it with 'r' */
>          sub[0] = 'r';
>          explicit_deny_rule = false;
> @@ -787,7 +788,7 @@ vah_add_path(virBufferPtr buf, const char *path, const char *perms, bool recursi
>      if (tmp[strlen(tmp) - 1] == '/')
>          tmp[strlen(tmp) - 1] = '\0';
>  
> -    virBufferAsprintf(buf, "  \"%s%s\" %s,\n", tmp, recursive ? "/**" : "", perms);
> +    virBufferAsprintf(buf, "  \"%s%s\" %s,\n", tmp, recursive ? "/**" : "", perms_new);
>      if (explicit_deny_rule) {
>          virBufferAddLit(buf, "  # don't audit writes to readonly files\n");
>          virBufferAsprintf(buf, "  deny \"%s%s\" w,\n", tmp, recursive ? "/**" : "");
> @@ -799,6 +800,7 @@ vah_add_path(virBufferPtr buf, const char *path, const char *perms, bool recursi
>  
>   cleanup:
>      VIR_FREE(tmp);
> +    VIR_FREE(perms_new);
>  
>      return rc;
>  }
> 




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