NFSv4: include MAY_WRITE in open permission mask for O_TRUNC

commit 5140f099ecd8a2f2808b7f7b720ee1bad8468974 upstream.

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.

BUG=b/539152172
TEST=presubmit
RELEASE_NOTE=Fixed CVE-2026-64298 in the Linux kernel.

cos-patch: security-moderate
Suggested-by: Trond Myklebust <trondmy@kernel.org>
Fixes: af22f94ae02a ("NFSv4: Simplify _nfs4_do_access()")
Cc: stable@vger.kernel.org
Change-Id: Iee2786eda1219add33fb6a9088bc5896d3fc35fc
Signed-off-by: Benjamin Coddington <bcodding@hammerspace.com>
Signed-off-by: Anna Schumaker <anna.schumaker@hammerspace.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kernel CVE Triage Automation <cloud-image-kernel-cve-triage-automation@prod.google.com>
Reviewed-on: https://cos-review.googlesource.com/c/third_party/kernel/+/176808
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
Reviewed-by: Dom Huh <domhuh@google.com>
Reviewed-by: Angel Adetula <angeladetula@google.com>