[PATCH v3] mtd: spi-nor: take the flash lock around spi_nor_restore()

[Itai Handler]

spi_nor_shutdown() and spi_nor_remove() call spi_nor_restore() without
nor->lock, which every other path to the chip takes through
spi_nor_prep_and_lock(). Both run with the MTD device still registered,
so another thread can be in the middle of an operation.

A busy flash ignores everything but status reads, so the restore is
silently dropped and the chip is left in 4-byte mode. A restore landing
between two chunks of a read switches the chip to 3-byte addressing
while spi_nor_read() carries on sending four address bytes.

Take the lock, so the restore runs between operations instead of during
one. This narrows the race rather than closing it: an operation starting
afterwards still addresses a 3-byte chip with nor->addr_nbytes left at 4.

Fixes: 59b356ffd0b0 ("mtd: m25p80: restore the status of SPI flash when exiting")
Cc: stable@xxxxxxxxxxxxxxx
Assisted-by: LLM
Signed-off-by: Itai Handler <itai.handler@xxxxxxxxx>
---
v2 1/3, the spi_nor_rww_start_exclusive() lock fix, is dropped from this
series now that it is queued in spi-nor/next. This patch still depends
on it: it adds the first ->shutdown and ->remove callers of the
exclusive lock, so an RWW flash would deadlock without it. Please let it
reach stable first.

Changes in v3:
- Fold the spi_nor_remove() patch into this one, as requested by
  Michael Walle. Both call sites now share a small helper.
- Cut the commit message down.
- Add an Assisted-by: tag, as requested by Michael Walle.
- Drop patch 1/3, queued in spi-nor/next.
- Link to v2:
  https://lore.kernel.org/r/20260914081149.1916589-1-itai.handler@xxxxxxxxx

 drivers/mtd/spi-nor/core.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/drivers/mtd/spi-nor/core.c b/drivers/mtd/spi-nor/core.c
index ccf4396cdcd0..c891197ffa5b 100644
--- a/drivers/mtd/spi-nor/core.c
+++ b/drivers/mtd/spi-nor/core.c
@@ -3849,11 +3849,26 @@ static int spi_nor_probe(struct spi_mem *spimem)
 				   data ? data->nr_parts : 0);
 }
 
+/*
+ * Restore between operations, not during one. Removal and shutdown both run
+ * with MTD users still attached: a busy flash silently ignores the commands
+ * spi_nor_restore() sends, and a restore landing inside a read changes the
+ * chip's address width under the transfer.
+ */
+static void spi_nor_restore_locked(struct spi_nor *nor)
+{
+	if (spi_nor_prep_and_lock(nor))
+		return;
+
+	spi_nor_restore(nor);
+	spi_nor_unlock_and_unprep(nor);
+}
+
 static int spi_nor_remove(struct spi_mem *spimem)
 {
 	struct spi_nor *nor = spi_mem_get_drvdata(spimem);
 
-	spi_nor_restore(nor);
+	spi_nor_restore_locked(nor);
 
 	/* Clean up MTD stuff. */
 	return mtd_device_unregister(&nor->mtd);
@@ -3863,7 +3878,7 @@ static void spi_nor_shutdown(struct spi_mem *spimem)
 {
 	struct spi_nor *nor = spi_mem_get_drvdata(spimem);
 
-	spi_nor_restore(nor);
+	spi_nor_restore_locked(nor);
 }
 
 /*

base-commit: 50d05c7c76c96b90462f24debacca971d2e86713
-- 
2.34.1