[PATCH v6 6/7] xfs: add permission check to free eofblocks ioctl

[Dwight Engen <dwight.engen@oracle.com>]

We need to check that userspace callers can only truncate preallocated
blocks from files they have write access to to prevent them from prematurley
reclaiming blocks from another user. The internal reclaimer will not specify
the internal XFS_KEOF_FLAGS_PERM_CHECK flag, but callers from userspace will
have it forced on.

Add check for read-only filesystem to free eofblocks ioctl.

Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
---
 fs/xfs/xfs_icache.c | 4 ++++
 fs/xfs/xfs_icache.h | 7 ++++++-
 fs/xfs/xfs_ioctl.c  | 3 +++
 3 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/fs/xfs/xfs_icache.c b/fs/xfs/xfs_icache.c
index ed35584..a80f38c 100644
--- a/fs/xfs/xfs_icache.c
+++ b/fs/xfs/xfs_icache.c
@@ -1247,6 +1247,10 @@ xfs_inode_free_eofblocks(
 		if (!xfs_inode_match_id(ip, eofb))
 			return 0;
 
+		if ((eofb->eof_flags & XFS_KEOF_FLAGS_PERM_CHECK) &&
+		    inode_permission(VFS_I(ip), MAY_WRITE))
+			return 0;
+
 		/* skip the inode if the file size is too small */
 		if (eofb->eof_flags & XFS_EOF_FLAGS_MINFILESIZE &&
 		    XFS_ISIZE(ip) < eofb->eof_min_file_size)
diff --git a/fs/xfs/xfs_icache.h b/fs/xfs/xfs_icache.h
index 66a1a22..0b52804 100644
--- a/fs/xfs/xfs_icache.h
+++ b/fs/xfs/xfs_icache.h
@@ -57,6 +57,11 @@ int xfs_inode_ag_iterator_tag(struct xfs_mount *mp,
 		int flags, void *args),
 	int flags, void *args, int tag);
 
+#define XFS_KEOF_FLAGS_PERM_CHECK	(1 << 31) /* check can write inode */
+#if XFS_KEOF_FLAGS_PERM_CHECK & XFS_EOF_FLAGS_VALID
+#error "Internal XFS_KEOF_FLAGS_PERM_CHECK duplicated bit from XFS_EOF_FLAGS_VALID"
+#endif
+
 static inline int
 xfs_fs_eofblocks_from_user(
 	struct xfs_fs_eofblocks		*src,
@@ -72,7 +77,7 @@ xfs_fs_eofblocks_from_user(
 	    memchr_inv(src->pad64, 0, sizeof(src->pad64)))
 		return EINVAL;
 
-	dst->eof_flags = src->eof_flags;
+	dst->eof_flags = src->eof_flags | XFS_KEOF_FLAGS_PERM_CHECK;
 	dst->eof_prid = src->eof_prid;
 	dst->eof_min_file_size = src->eof_min_file_size;
 
diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
index 6e72eff..8500507 100644
--- a/fs/xfs/xfs_ioctl.c
+++ b/fs/xfs/xfs_ioctl.c
@@ -1613,6 +1613,9 @@ xfs_file_ioctl(
 		struct xfs_fs_eofblocks eofb;
 		struct xfs_eofblocks keofb;
 
+		if (IS_RDONLY(inode))
+			return -XFS_ERROR(EROFS);
+
 		if (copy_from_user(&eofb, arg, sizeof(eofb)))
 			return -XFS_ERROR(EFAULT);
 
-- 
1.8.1.4

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