Reproducible crash in kAFS with just "cp -a"

[David Howells]

Gaja Sophie Peters <gaja.peters at math.uni-hamburg.de> wrote:

> Already in earlier kernel-versions I got occasional crashes, but today I
> could reproduce the crash pretty well in both Kernel versions
> "5.8.0.44.50~20.04.30" (the current "hardware-enablement"-Kernel of
> Ubuntu) and also in Kernel 5.11.0-051100.202102142330 (the newest
> "mainline"-Kernel of Ubuntu).
> 
> The only four commands I entered are
> 
> 	keyctl new_session
> 	kinit <account>
> 	aklog-kafs
> 	cp -a /afs/some.openafs.cell/some.file /tmp

This seems to be reproducible with the getfattr command also.

Would it be possible for you to test the attached patch?

Thanks,
David
---
afs: Fix accessing YFS xattrs on a non-YFS server

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.

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>
---

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 = {