[PATCH v2 1/2] afs: Fix accessing YFS xattrs on a non-YFS server

[Jeffrey E Altman]

Reviewed-by: Jeffrey Altman <jaltman at auristor.com>

On 3/11/2021 6:06 PM, David Howells (dhowells at redhat.com) wrote:
> If someone attempts to access YFS-related xattrs (e.g. afs.yfs.acl) on a
> file on a non-YFS AFS server (such as OpenAFS), then the kernel will jump
> to a NULL function pointer because the afs_fetch_acl_operation descriptor
> doesn't point to a function for issuing an operation on a non-YFS
> server[1].
> 
> Fix this by making afs_wait_for_operation() check that the issue_afs_rpc
> method is set before jumping to it and setting -ENOTSUPP if not.  This fix
> also covers other potential operations that also only exist on YFS servers.
> 
> afs_xattr_get/set_yfs() then need to translate -ENOTSUPP to -ENODATA as the
> former error is internal to the kernel.
> 
> The bug shows up as an oops like the following:
> 
> 	BUG: kernel NULL pointer dereference, address: 0000000000000000
> 	[...]
> 	Code: Unable to access opcode bytes at RIP 0xffffffffffffffd6.
> 	[...]
> 	Call Trace:
> 	 afs_wait_for_operation+0x83/0x1b0 [kafs]
> 	 afs_xattr_get_yfs+0xe6/0x270 [kafs]
> 	 __vfs_getxattr+0x59/0x80
> 	 vfs_getxattr+0x11c/0x140
> 	 getxattr+0x181/0x250
> 	 ? __check_object_size+0x13f/0x150
> 	 ? __fput+0x16d/0x250
> 	 __x64_sys_fgetxattr+0x64/0xb0
> 	 do_syscall_64+0x49/0xc0
> 	 entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 	RIP: 0033:0x7fb120a9defe
> 
> This was triggered with "cp -a" which attempts to copy xattrs, including
> afs ones, but is easier to reproduce with getfattr, e.g.:
> 
> 	getfattr -d -m ".*" /afs/openafs.org/
> 
> Fixes: e49c7b2f6de7 ("afs: Build an abstraction around an "operation" concept")
> Reported-by: Gaja Sophie Peters <gaja.peters at math.uni-hamburg.de>
> Signed-off-by: David Howells <dhowells at redhat.com>
> Tested-by: Gaja Sophie Peters <gaja.peters at math.uni-hamburg.de>
> cc: linux-afs at lists.infradead.org
> Link: http://lists.infradead.org/pipermail/linux-afs/2021-March/003498.html [1]
> ---
> 
>   fs/afs/fs_operation.c |    7 +++++--
>   fs/afs/xattr.c        |    8 +++++++-
>   2 files changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/afs/fs_operation.c b/fs/afs/fs_operation.c
> index 97cab12b0a6c..71c58723763d 100644
> --- a/fs/afs/fs_operation.c
> +++ b/fs/afs/fs_operation.c
> @@ -181,10 +181,13 @@ void afs_wait_for_operation(struct afs_operation *op)
>   		if (test_bit(AFS_SERVER_FL_IS_YFS, &op->server->flags) &&
>   		    op->ops->issue_yfs_rpc)
>   			op->ops->issue_yfs_rpc(op);
> -		else
> +		else if (op->ops->issue_afs_rpc)
>   			op->ops->issue_afs_rpc(op);
> +		else
> +			op->ac.error = -ENOTSUPP;
>   
> -		op->error = afs_wait_for_call_to_complete(op->call, &op->ac);
> +		if (op->call)
> +			op->error = afs_wait_for_call_to_complete(op->call, &op->ac);
>   	}
>   
>   	switch (op->error) {
> diff --git a/fs/afs/xattr.c b/fs/afs/xattr.c
> index c629caae5002..4934e325a14a 100644
> --- a/fs/afs/xattr.c
> +++ b/fs/afs/xattr.c
> @@ -231,6 +231,8 @@ static int afs_xattr_get_yfs(const struct xattr_handler *handler,
>   			else
>   				ret = -ERANGE;
>   		}
> +	} else if (ret == -ENOTSUPP) {
> +		ret = -ENODATA;
>   	}
>   
>   error_yacl:
> @@ -256,6 +258,7 @@ static int afs_xattr_set_yfs(const struct xattr_handler *handler,
>   {
>   	struct afs_operation *op;
>   	struct afs_vnode *vnode = AFS_FS_I(inode);
> +	int ret;
>   
>   	if (flags == XATTR_CREATE ||
>   	    strcmp(name, "acl") != 0)
> @@ -270,7 +273,10 @@ static int afs_xattr_set_yfs(const struct xattr_handler *handler,
>   		return afs_put_operation(op);
>   
>   	op->ops = &yfs_store_opaque_acl2_operation;
> -	return afs_do_sync_operation(op);
> +	ret = afs_do_sync_operation(op);
> +	if (ret == -ENOTSUPP)
> +		ret = -ENODATA;
> +	return ret;
>   }
>   
>   static const struct xattr_handler afs_xattr_yfs_handler = {
> 
> 
> 
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> http://lists.infradead.org/mailman/listinfo/linux-afs
> 
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