[PATCH v2 1/2] pidfs: do not report fields that do not fit into the caller's struct

[Nickolai Zeldovich <nickolai@csail.mit.edu>]

The documentation of struct pidfd_info promises that "If the structure
size is too small to contain a field (requested or not), to avoid
confusion the mask will not contain a bit for that field", and asks
userspace to trust the returned mask when deciding which fields hold
valid data.

pidfd_info() does not keep that promise. It builds kinfo.mask without
looking at the size encoded in the ioctl command, and
copy_struct_to_user() then truncates the copy to that size. A caller
that passes a 64-byte struct (PIDFD_INFO_SIZE_VER0) and requests
PIDFD_INFO_COREDUMP or PIDFD_INFO_SUPPORTED_MASK gets a mask with
PIDFD_INFO_COREDUMP, PIDFD_INFO_COREDUMP_SIGNAL, PIDFD_INFO_COREDUMP_CODE
and PIDFD_INFO_SUPPORTED_MASK set, while the coredump_* and
supported_mask fields behind the 64 bytes were never written. Trusting
the mask as documented, userspace then reads whatever its own buffer
held at those offsets. The same happens for a 72- or 80-byte struct
and the fields added after it.

Clear the bits of the fields that lie beyond the caller's size before
the copy, in the one place that knows both the final mask and the size.
A request for a field that cannot be returned is thereby answered the
way the header describes: the field is simply absent from the mask.

Fixes: 1d8db6fd698d ("pidfs, coredump: add PIDFD_INFO_COREDUMP")
Signed-off-by: Nickolai Zeldovich <nickolai@csail.mit.edu>
---

Notes:
    The three size thresholds follow the field layout: coredump_mask and
    coredump_signal end at PIDFD_INFO_SIZE_VER1 (72), coredump_code at
    PIDFD_INFO_SIZE_VER2 (80), supported_mask at PIDFD_INFO_SIZE_VER3 (88).
    Masking the request bits early instead (before the coredump and
    supported_mask blocks run) would save a little work for such callers
    but leaves PIDFD_INFO_COREDUMP set by the dumpable-state fallback for a
    live task; clearing the result once before copy_struct_to_user() covers
    every way a bit can be set.

 fs/pidfs.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/fs/pidfs.c b/fs/pidfs.c
index 7f8c323ce6cb8..cdb0916eae71b 100644
--- a/fs/pidfs.c
+++ b/fs/pidfs.c
@@ -485,6 +485,14 @@ static long pidfd_info(struct file *file, unsigned int cmd, unsigned long arg)
 		kinfo.supported_mask = PIDFD_INFO_SUPPORTED;
 	}
 
+	/* Only report fields that fit into the caller's struct. */
+	if (usize < PIDFD_INFO_SIZE_VER1)
+		kinfo.mask &= ~(PIDFD_INFO_COREDUMP | PIDFD_INFO_COREDUMP_SIGNAL);
+	if (usize < PIDFD_INFO_SIZE_VER2)
+		kinfo.mask &= ~PIDFD_INFO_COREDUMP_CODE;
+	if (usize < PIDFD_INFO_SIZE_VER3)
+		kinfo.mask &= ~PIDFD_INFO_SUPPORTED_MASK;
+
 	/* Are there bits in the return mask not present in PIDFD_INFO_SUPPORTED? */
 	WARN_ON_ONCE(~PIDFD_INFO_SUPPORTED & kinfo.mask);
 	/*
-- 
2.53.0