NFSv4: include MAY_WRITE in open permission mask for O_TRUNC

POSIX requires write permission to truncate a file, so an open() that
specifies O_TRUNC must be authorized for write access regardless of the
O_ACCMODE access mode.

nfs_open_permission_mask() builds the access mask passed to
nfs_may_open(), which is the local authorization gate for OPENs the
client serves itself from a cached write delegation via the
can_open_delegated() path in nfs4_try_open_cached().  The mask is
derived from O_ACCMODE alone, so an open(O_RDONLY | O_TRUNC) against a
file the caller cannot write requests only MAY_READ and passes the
local check.  The OPEN is then satisfied locally and the truncation is
issued to the server as a SETATTR(size=0) over the delegation stateid,
which the server accepts under standard write-delegation semantics.
POSIX requires that this open fail with EACCES.

Include MAY_WRITE in the mask whenever O_TRUNC is set so the local
check matches the access the server would have enforced.

Suggested-by: Trond Myklebust <trondmy@kernel.org>
Fixes: af22f94ae0 ("NFSv4: Simplify _nfs4_do_access()")
Cc: stable@vger.kernel.org
Signed-off-by: Benjamin Coddington <bcodding@hammerspace.com>
Signed-off-by: Anna Schumaker <anna.schumaker@hammerspace.com>

fs/nfs/dir.c

@@ -3346,6 +3346,8 @@ static int nfs_open_permission_mask(int openflags)
 			mask |= MAY_READ;
 		if ((openflags & O_ACCMODE) != O_RDONLY)
 			mask |= MAY_WRITE;
+		if (openflags & O_TRUNC)
+			mask |= MAY_WRITE;
 	}
 
 	return mask;