linux-kernel - Re: [PATCH] EVM: Fix null dereference on xattr when xattr fails to allocate

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Date:   Tue, 29 May 2018 08:31:32 -0400
From:   Mimi Zohar <zohar@...ux.vnet.ibm.com>
To:     Dan Carpenter <dan.carpenter@...cle.com>,
        Colin King <colin.king@...onical.com>
Cc:     Matthew Garrett <mjg59@...gle.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] EVM: Fix null dereference on xattr when xattr fails to
 allocate

Hi Dan,

On Tue, 2018-05-29 at 12:05 +0300, Dan Carpenter wrote:
> Not really related to this patch except I was looking at the function:
> 
> security/integrity/evm/evm_secfs.c
>    191          ab = audit_log_start(NULL, GFP_KERNEL, AUDIT_INTEGRITY_EVM_XATTR);
>    192          if (IS_ERR(ab))
>    193                  return PTR_ERR(ab);
>    194  
>    195          xattr = kmalloc(sizeof(struct xattr_list), GFP_KERNEL);
>    196          if (!xattr) {
>    197                  err = -ENOMEM;
>    198                  goto out;
>    199          }
>    200  
>    201          xattr->name = memdup_user_nul(buf, count);
>    202          if (IS_ERR(xattr->name)) {
>    203                  err = PTR_ERR(xattr->name);
>    204                  xattr->name = NULL;
>    205                  goto out;
>    206          }
>    207  
>    208          /* Remove any trailing newline */
>    209          len = strlen(xattr->name);
>    210          if (xattr->name[len-1] == '\n')
> 
> strlen() could be zero, leading to a read underflow here.

Thanks!  Could you modify the maximum xattr size check (before this
code snippet) to check for underflow?

Mimi


> 
>    211                  xattr->name[len-1] = '\0';
>    212  
>    213          if (strcmp(xattr->name, ".") == 0) {
>    214                  evm_xattrs_locked = 1;
>    215                  newattrs.ia_mode = S_IFREG | 0440;
>    216                  newattrs.ia_valid = ATTR_MODE;
>    217                  inode = evm_xattrs->d_inode;
>    218                  inode_lock(inode);
>    219                  err = simple_setattr(evm_xattrs, &newattrs);
>    220                  inode_unlock(inode);
>    221                  audit_log_format(ab, "locked");
>    222                  if (!err)
>    223                          err = count;
>    224                  goto out;
>    225          }
> 
> regards,
> dan carpenter
>