[PATCH] fs: fix xattr permission checking error

[PATCH] fs: fix xattr permission checking error

[Nicolas Belouin]

Fix an issue making trusted xattr world readable and other
cap_sys_admin only

Signed-off-by: Nicolas Belouin <nicolas@belouin.fr>
---
 fs/hfsplus/xattr.c | 2 +-
 fs/jfs/xattr.c     | 5 ++---
 2 files changed, 3 insertions(+), 4 deletions(-)

diff --git a/fs/hfsplus/xattr.c b/fs/hfsplus/xattr.c
index d37bb88dc746..ae03a19196ef 100644
--- a/fs/hfsplus/xattr.c
+++ b/fs/hfsplus/xattr.c
@@ -604,7 +604,7 @@ static inline int can_list(const char *xattr_name)
 	if (!xattr_name)
 		return 0;
 
-	return strncmp(xattr_name, XATTR_TRUSTED_PREFIX,
+	return !strncmp(xattr_name, XATTR_TRUSTED_PREFIX,
 			XATTR_TRUSTED_PREFIX_LEN) ||
 				capable(CAP_SYS_ADMIN);
 }
diff --git a/fs/jfs/xattr.c b/fs/jfs/xattr.c
index c60f3d32ee91..1c46573a96ed 100644
--- a/fs/jfs/xattr.c
+++ b/fs/jfs/xattr.c
@@ -858,9 +858,8 @@ ssize_t __jfs_getxattr(struct inode *inode, const char *name, void *data,
  */
 static inline int can_list(struct jfs_ea *ea)
 {
-	return (strncmp(ea->name, XATTR_TRUSTED_PREFIX,
-			    XATTR_TRUSTED_PREFIX_LEN) ||
-		capable(CAP_SYS_ADMIN));
+	return (!strncmp(ea->name, XATTR_TRUSTED_PREFIX,
+			 XATTR_TRUSTED_PREFIX_LEN) || capable(CAP_SYS_ADMIN));
 }
 
 ssize_t jfs_listxattr(struct dentry * dentry, char *data, size_t buf_size)
-- 
2.14.2

Re: [PATCH] fs: fix xattr permission checking error

[Andreas Dilger]

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On Oct 21, 2017, at 7:39 AM, Nicolas Belouin <nicolas@belouin.fr> wrote:
> 
> Fix an issue making trusted xattr world readable and other
> cap_sys_admin only
> 
> Signed-off-by: Nicolas Belouin <nicolas@belouin.fr>
> ---
> fs/hfsplus/xattr.c | 2 +-
> fs/jfs/xattr.c     | 5 ++---
> 2 files changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/hfsplus/xattr.c b/fs/hfsplus/xattr.c
> index d37bb88dc746..ae03a19196ef 100644
> --- a/fs/hfsplus/xattr.c
> +++ b/fs/hfsplus/xattr.c
> @@ -604,7 +604,7 @@ static inline int can_list(const char *xattr_name)
> 	if (!xattr_name)
> 		return 0;
> 
> -	return strncmp(xattr_name, XATTR_TRUSTED_PREFIX,
> +	return !strncmp(xattr_name, XATTR_TRUSTED_PREFIX,
> 			XATTR_TRUSTED_PREFIX_LEN) ||
> 				capable(CAP_SYS_ADMIN);

I don't think this is correct.  This means "you can list the xattr if
it IS 'trusted.*', OR if you have sysadmin privilege", so non-trusted
xattrs could not be listed by regular users.

As can be seen by this defect, the use of "strncmp()" with an explicit
boolean return code is confusing and subject to errors, in particular
"strncmp()" returning 0 (false) means the strings MATCH.  My preference
is to explicitly check "strncmp() == 0" for the match, as this is more
clear to the reader that strncmp() has a non-standard return convention.

To my reading, the original logic is correct, which is "you can list
the xattr if it is not 'trusted.*' OR if you have sysadmin privilege",
but it could be improved like:

	return strncmp(xattr_name, XATTR_TRUSTED_PREFIX,
		       XATTR_TRUSTED_PREFIX_LEN) != 0 ||
		capable(CAP_SYS_ADMIN);

> diff --git a/fs/jfs/xattr.c b/fs/jfs/xattr.c
> index c60f3d32ee91..1c46573a96ed 100644
> --- a/fs/jfs/xattr.c
> +++ b/fs/jfs/xattr.c
> @@ -858,9 +858,8 @@ ssize_t __jfs_getxattr(struct inode *inode, const char *name, void *data,
>  */
> static inline int can_list(struct jfs_ea *ea)
> {
> -	return (strncmp(ea->name, XATTR_TRUSTED_PREFIX,
> -			    XATTR_TRUSTED_PREFIX_LEN) ||
> -		capable(CAP_SYS_ADMIN));
> +	return (!strncmp(ea->name, XATTR_TRUSTED_PREFIX,
> +			 XATTR_TRUSTED_PREFIX_LEN) || capable(CAP_SYS_ADMIN));
> }

I think the original code is also correct here, and your patch is adding
a bug.

Cheers, Andreas






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Re: [PATCH] fs: fix xattr permission checking error

[Nicolas Belouin]

Indeed, I was mistaken here.
Thanks for the review.
I will do the "strncmp() ==0" in another patch as it is particularly unclear in this context.

On October 21, 2017 9:48:16 PM GMT+02:00, Andreas Dilger <adilger@dilger.ca> wrote:
>On Oct 21, 2017, at 7:39 AM, Nicolas Belouin <nicolas@belouin.fr>
>wrote:
>> 
>> Fix an issue making trusted xattr world readable and other
>> cap_sys_admin only
>> 
>> Signed-off-by: Nicolas Belouin <nicolas@belouin.fr>
>> ---
>> fs/hfsplus/xattr.c | 2 +-
>> fs/jfs/xattr.c     | 5 ++---
>> 2 files changed, 3 insertions(+), 4 deletions(-)
>> 
>> diff --git a/fs/hfsplus/xattr.c b/fs/hfsplus/xattr.c
>> index d37bb88dc746..ae03a19196ef 100644
>> --- a/fs/hfsplus/xattr.c
>> +++ b/fs/hfsplus/xattr.c
>> @@ -604,7 +604,7 @@ static inline int can_list(const char
>*xattr_name)
>> 	if (!xattr_name)
>> 		return 0;
>> 
>> -	return strncmp(xattr_name, XATTR_TRUSTED_PREFIX,
>> +	return !strncmp(xattr_name, XATTR_TRUSTED_PREFIX,
>> 			XATTR_TRUSTED_PREFIX_LEN) ||
>> 				capable(CAP_SYS_ADMIN);
>
>I don't think this is correct.  This means "you can list the xattr if
>it IS 'trusted.*', OR if you have sysadmin privilege", so non-trusted
>xattrs could not be listed by regular users.
>
>As can be seen by this defect, the use of "strncmp()" with an explicit
>boolean return code is confusing and subject to errors, in particular
>"strncmp()" returning 0 (false) means the strings MATCH.  My preference
>is to explicitly check "strncmp() == 0" for the match, as this is more
>clear to the reader that strncmp() has a non-standard return
>convention.
>
>To my reading, the original logic is correct, which is "you can list
>the xattr if it is not 'trusted.*' OR if you have sysadmin privilege",
>but it could be improved like:
>
>	return strncmp(xattr_name, XATTR_TRUSTED_PREFIX,
>		       XATTR_TRUSTED_PREFIX_LEN) != 0 ||
>		capable(CAP_SYS_ADMIN);
>
>> diff --git a/fs/jfs/xattr.c b/fs/jfs/xattr.c
>> index c60f3d32ee91..1c46573a96ed 100644
>> --- a/fs/jfs/xattr.c
>> +++ b/fs/jfs/xattr.c
>> @@ -858,9 +858,8 @@ ssize_t __jfs_getxattr(struct inode *inode, const
>char *name, void *data,
>>  */
>> static inline int can_list(struct jfs_ea *ea)
>> {
>> -	return (strncmp(ea->name, XATTR_TRUSTED_PREFIX,
>> -			    XATTR_TRUSTED_PREFIX_LEN) ||
>> -		capable(CAP_SYS_ADMIN));
>> +	return (!strncmp(ea->name, XATTR_TRUSTED_PREFIX,
>> +			 XATTR_TRUSTED_PREFIX_LEN) || capable(CAP_SYS_ADMIN));
>> }
>
>I think the original code is also correct here, and your patch is
>adding
>a bug.
>
>Cheers, Andreas

Nicolas

Re: [PATCH] fs: fix xattr permission checking error

[Theodore Ts'o]

On Sat, Oct 21, 2017 at 03:39:47PM +0200, Nicolas Belouin wrote:
> Fix an issue making trusted xattr world readable and other
> cap_sys_admin only
>

NACK.  It is *documented* that trusted xattrs are only supposed to be
readable by root.

					- Ted