[PATCH 0/2] mtd: spi-nor: fix the unlocked restore on shutdown and remove

[Itai Handler]

Two threads can talk to the flash at once during reboot/kexec and during
an unbind, because spi_nor_restore() is called without nor->lock while
MTD users are still attached. A busy flash silently ignores the restore
and is left in 4-byte addressing, which is exactly the failure the
restore was added to prevent; a restore landing inside a read corrupts
the rest of that read instead.

I reproduced this under QEMU, using a flash model extended to implement
erase busy time. With an erase outstanding, spi_nor_shutdown() issues
WREN, EX4B and WRDI; the chip refuses all three because it is busy, and
each one still reports success to the caller:

  the flash was mid-erase when spi_nor_shutdown() tried to put it back
  into 3-byte addressing, and refused 3 of its command(s):
  0x06 (four_byte=1), 0xe9 (four_byte=1), 0x04 (four_byte=1)

four_byte=1 is the mode the chip was left in, and so the mode the next
kernel inherits. With the patch, shutdown waits for the erase and the
restore reaches an idle chip. The reproduction is deterministic: the
workload keeps the flash busy continuously, so no timing window is
involved.

Two caveats on that. It runs on a 5.10 vendor tree rather than mainline,
though the path is unchanged - mainline's spi_nor_shutdown() has the
same unlocked spi_nor_restore() call. And what started the investigation
was intermittent hangs after kexec on a Zynq UltraScale+ board, which I
have not tied to this race.

Patch 1 fixes ->shutdown, which every reboot and kexec goes through,
and is marked for stable. Patch 2 fixes the identical problem in ->remove;
I have deliberately not marked it for stable, since nobody has reported
hitting it and it changes how long an unbind can block.

Note what patch 1 does not do: the restore still runs with MTD users
attached, so an operation starting after it completes still addresses a
3-byte chip with nor->addr_nbytes == 4. Serialising against operations
already in flight is what stops the restore being issued into a busy
chip; fully closing the window would mean stopping MTD from accepting
operations before ->shutdown, which seemed too big a change to fold in
here. I am happy to look at that separately if you would prefer it.

The patches are independent; patch 2 can be dropped without affecting
patch 1.

Itai Handler (2):
  mtd: spi-nor: take the flash lock in spi_nor_shutdown()
  mtd: spi-nor: take the flash lock in spi_nor_remove()

 drivers/mtd/spi-nor/core.c | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)