[PATCH v3 17/33] swim: Fix buffer overflow

[Finn Thain <fthain@linux-m68k.org>]

The effect of this bug can be observed as swim_read_sector_data()
inexplicably returning -5, or an error flag indicating that a mark byte
was read from the data register, or other odd behviour.

When copying bytes from the chip FIFO to the read buffer, the driver
keeps count of the remaining buffer space using register %d4. A counter
in register %d2 serves as a timeout. The driver polls (%a2), the handshake
register, until flags indicate that byte(s) have arrived in the FIFO.

        movel   #sector_size-1, %d4
read_new_data:
        movew   #max_retry, %d2
read_data_loop:
        moveb   %a2@, %d5
        andb    #0xc0, %d5
        dbne    %d2, read_data_loop
        beq     data_exit
        moveb   %a5@, %a4@+
        andb    #0x40, %d5
        dbne    %d4, read_new_data
        beq     exit_loop

Note that the exit_loop branch depends upon a flag in the handshake
register and not on the remaining buffer space. Hence there may be no
branch to exit_loop after %d4 is decremented to -1 (i.e. full buffer).

        moveb   %a5@, %a4@+
        dbra    %d4, read_new_data
exit_loop:

Here is a second decrement of %d4 which can now reach -2. But the buffer
bounds check is a comparison with -1, which is now ineffective. Hence the
loop will continue copying until %d2 eventually reaches -1.

Fix this bug by terminating the loop as soon as %d4 or %d2 reach -1.
Reset the timeout whenever a byte is copied.

Fixes: 8852ecd97488 ("m68k: mac - Add SWIM floppy support")
Signed-off-by: Finn Thain <fthain@linux-m68k.org>
---
Changed since v1:
 - Avoid jumping to a redundant AND.B.
 - Avoid a second handshake register access when there's already a byte in
 the FIFO.

Changed since v2:
 - Dropped reviewed-by tag due to unreviewed changes made since v1.
---
 drivers/block/swim_asm.S | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/drivers/block/swim_asm.S b/drivers/block/swim_asm.S
index 699f7c90dd1c..e06aadb411a1 100644
--- a/drivers/block/swim_asm.S
+++ b/drivers/block/swim_asm.S
@@ -43,6 +43,8 @@
 	.equ	sector_size, 512
 
 	.equ	.Lhr_crc_error,		0x02
+	.equ	.Lhr_fifo_2bytes,	0x40
+	.equ	.Lhr_fifo_1byte,	0x80
 
 	.global swim_read_sector_header
 swim_read_sector_header:
@@ -189,20 +191,20 @@ wait_data_mark_byte:
 	/* read data */
 
 	movel	#sector_size-1, %d4		/* sector size */
-read_new_data:
 	movew	#max_retry, %d2
 read_data_loop:
 	moveb	%a2@, %d5
-	andb	#0xc0, %d5
+	andb	#(.Lhr_fifo_1byte + .Lhr_fifo_2bytes), %d5
 	dbne	%d2, read_data_loop
 	beq	data_exit
+	moveq	#max_retry, %d2
 	moveb	%a5@, %a4@+
-	andb	#0x40, %d5
-	dbne	%d4, read_new_data
-	beq	exit_loop
+	dbra	%d4, 1f
+	bra	data_crc0
+1:	andb	#.Lhr_fifo_2bytes, %d5
+	beq	read_data_loop
 	moveb	%a5@, %a4@+
-	dbra	%d4, read_new_data
-exit_loop:
+	dbra	%d4, read_data_loop
 
 	/* read CRC */
 
-- 
2.52.0