mtd: spi-nor: Don't copy self-pointing struct around

spi_nor_parse_sfdp() modifies the passed structure so that it points to
itself (params.erase_map.regions to params.erase_map.uniform_region). This
makes it impossible to copy the local struct anywhere else.

Therefore only use memcpy() in backup-restore scenario. The bug may show up
like below:

BUG: unable to handle page fault for address: ffffc90000b377f8
Oops: 0000 [#1] PREEMPT SMP NOPTI
CPU: 4 PID: 3500 Comm: flashcp Tainted: G           O      5.4.53-... #1
...
RIP: 0010:spi_nor_erase+0x8e/0x5c0
Code: 64 24 18 89 db 4d 8b b5 d0 04 00 00 4c 89 64 24 18 4c 89 64 24 20 eb 12 a8 10 0f 85 59 02 00 00 49 83 c6 10 0f 84 4f 02 00 00 <49> 8b 06 48 89 c2 48 83 e2 c0 48 89 d1 49 03 4e 08 48 39 cb 73 d8
RSP: 0018:ffffc9000217fc48 EFLAGS: 00010206
RAX: 0000000000740000 RBX: 0000000000000000 RCX: 0000000000740000
RDX: ffff8884550c9980 RSI: ffff88844f9c0bc0 RDI: ffff88844ede7bb8
RBP: 0000000000740000 R08: ffffffff815bfbe0 R09: ffff88844f9c0bc0
R10: 0000000000000000 R11: 0000000000000000 R12: ffffc9000217fc60
R13: ffff88844ede7818 R14: ffffc90000b377f8 R15: 0000000000000000
FS:  00007f4699780500(0000) GS:ffff88846ff00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffc90000b377f8 CR3: 00000004538ee000 CR4: 0000000000340fe0
Call Trace:
 part_erase+0x27/0x50
 mtdchar_ioctl+0x831/0xba0
 ? filemap_map_pages+0x186/0x3d0
 ? do_filp_open+0xad/0x110
 ? _copy_to_user+0x22/0x30
 ? cp_new_stat+0x150/0x180
 mtdchar_unlocked_ioctl+0x2a/0x40
 do_vfs_ioctl+0xa0/0x630
 ? __do_sys_newfstat+0x3c/0x60
 ksys_ioctl+0x70/0x80
 __x64_sys_ioctl+0x16/0x20
 do_syscall_64+0x6a/0x200
 ? prepare_exit_to_usermode+0x50/0xd0
 entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x7f46996b6817

Cc: stable@vger.kernel.org
Fixes: c4687217 ("mtd: spi-nor: Move erase_map to 'struct spi_nor_flash_parameter'")
Co-developed-by: Matija Glavinic Pecotic <matija.glavinic-pecotic.ext@nokia.com>
Signed-off-by: Matija Glavinic Pecotic <matija.glavinic-pecotic.ext@nokia.com>
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Tested-by: Baurzhan Ismagulov <ibr@radix50.net>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20201005084803.23460-1-alexander.sverdlin@nokia.com

drivers/mtd/spi-nor/core.c

@@ -2701,11 +2701,10 @@ static void spi_nor_sfdp_init_params(struct spi_nor *nor)
 
 	memcpy(&sfdp_params, nor->params, sizeof(sfdp_params));
 
-	if (spi_nor_parse_sfdp(nor, &sfdp_params)) {
+	if (spi_nor_parse_sfdp(nor, nor->params)) {
+		memcpy(nor->params, &sfdp_params, sizeof(*nor->params));
 		nor->addr_width = 0;
 		nor->flags &= ~SNOR_F_4B_OPCODES;
-	} else {
-		memcpy(nor->params, &sfdp_params, sizeof(*nor->params));
 	}
 }