mtd: core: Fix freeing of otp_info buffer

Commit 4b361cf ("mtd: core: add OTP nvmem provider support") is
causing the following panic ...

 ------------[ cut here ]------------
 kernel BUG at /local/workdir/tegra/linux_next/kernel/mm/slab.c:2730!
 Internal error: Oops - BUG: 0 [#1] PREEMPT SMP ARM
 Modules linked in:
 CPU: 3 PID: 1 Comm: swapper/0 Not tainted 5.13.0-rc2-next-20210518 #1
 Hardware name: NVIDIA Tegra SoC (Flattened Device Tree)
 PC is at ___cache_free+0x3f8/0x51c
 ...
 [<c029bb1c>] (___cache_free) from [<c029c658>] (kfree+0xac/0x1bc)
 [<c029c658>] (kfree) from [<c06da094>] (mtd_otp_size+0xc4/0x108)
 [<c06da094>] (mtd_otp_size) from [<c06dc864>] (mtd_device_parse_register+0xe4/0x2b4)
 [<c06dc864>] (mtd_device_parse_register) from [<c06e3ccc>] (spi_nor_probe+0x210/0x2c0)
 [<c06e3ccc>] (spi_nor_probe) from [<c06e9578>] (spi_probe+0x88/0xac)
 [<c06e9578>] (spi_probe) from [<c066891c>] (really_probe+0x214/0x3a4)
 [<c066891c>] (really_probe) from [<c0668b14>] (driver_probe_device+0x68/0xc0)
 [<c0668b14>] (driver_probe_device) from [<c0666cf8>] (bus_for_each_drv+0x5c/0xbc)
 [<c0666cf8>] (bus_for_each_drv) from [<c0668694>] (__device_attach+0xe4/0x150)
 [<c0668694>] (__device_attach) from [<c06679e0>] (bus_probe_device+0x84/0x8c)
 [<c06679e0>] (bus_probe_device) from [<c06657f8>] (device_add+0x48c/0x868)
 [<c06657f8>] (device_add) from [<c06eb784>] (spi_add_device+0xa0/0x168)
 [<c06eb784>] (spi_add_device) from [<c06ec9a8>] (spi_register_controller+0x8b8/0xb38)
 [<c06ec9a8>] (spi_register_controller) from [<c06ecc3c>] (devm_spi_register_controller+0x14/0x50)
 [<c06ecc3c>] (devm_spi_register_controller) from [<c06f0510>] (tegra_spi_probe+0x33c/0x450)
 [<c06f0510>] (tegra_spi_probe) from [<c066abec>] (platform_probe+0x5c/0xb8)
 [<c066abec>] (platform_probe) from [<c066891c>] (really_probe+0x214/0x3a4)
 [<c066891c>] (really_probe) from [<c0668b14>] (driver_probe_device+0x68/0xc0)
 [<c0668b14>] (driver_probe_device) from [<c0668e30>] (device_driver_attach+0x58/0x60)
 [<c0668e30>] (device_driver_attach) from [<c0668eb8>] (__driver_attach+0x80/0xc8)
 [<c0668eb8>] (__driver_attach) from [<c0666c48>] (bus_for_each_dev+0x78/0xb8)
 [<c0666c48>] (bus_for_each_dev) from [<c0667c44>] (bus_add_driver+0x164/0x1e8)
 [<c0667c44>] (bus_add_driver) from [<c066997c>] (driver_register+0x7c/0x114)
 [<c066997c>] (driver_register) from [<c010223c>] (do_one_initcall+0x50/0x2b0)
 [<c010223c>] (do_one_initcall) from [<c11011f0>] (kernel_init_freeable+0x1a8/0x1fc)
 [<c11011f0>] (kernel_init_freeable) from [<c0c09190>] (kernel_init+0x8/0x118)
 [<c0c09190>] (kernel_init) from [<c01001b0>] (ret_from_fork+0x14/0x24)
 ...
 ---[ end trace 0f652dd222de75d7 ]---

In the function mtd_otp_size() a buffer is allocated by calling
kmalloc() and a pointer to the buffer is stored in a variable 'info'.
The pointer 'info' may then be incremented depending on the length
returned from mtd_get_user/fact_prot_info(). If 'info' is incremented,
when kfree() is called to free the buffer the above panic occurs because
we are no longer passing the original address of the buffer allocated.
Fix this by indexing through the buffer allocated to avoid incrementing
the pointer.

Fixes: 4b361cf ("mtd: core: add OTP nvmem provider support")
Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Reviewed-by: Michael Walle <michael@walle.cc>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210518185503.162787-1-jonathanh@nvidia.com

drivers/mtd/mtdcore.c

@@ -792,10 +792,8 @@ static ssize_t mtd_otp_size(struct mtd_info *mtd, bool is_user)
 	if (ret)
 		goto err;
 
-	for (i = 0; i < retlen / sizeof(*info); i++) {
-		size += info->length;
-		info++;
-	}
+	for (i = 0; i < retlen / sizeof(*info); i++)
+		size += info[i].length;
 
 	kfree(info);
 	return size;