Re: [PATCH 1/2] mtdchar: prevent integer overflow in a safety check

[Richard Weinberger <richard@nod.at>]

----- Ursprüngliche Mail -----
> Von: "Michał Kępień" <kernel@kempniu.pl>
> An: "Miquel Raynal" <miquel.raynal@bootlin.com>, "richard" <richard@nod.at>, "Vignesh Raghavendra" <vigneshr@ti.com>
> CC: "linux-mtd" <linux-mtd@lists.infradead.org>, "linux-kernel" <linux-kernel@vger.kernel.org>
> Gesendet: Montag, 16. Mai 2022 09:06:00
> Betreff: [PATCH 1/2] mtdchar: prevent integer overflow in a safety check

> Commit 6420ac0af95d ("mtdchar: prevent unbounded allocation in MEMWRITE
> ioctl") added a safety check to mtdchar_write_ioctl() which attempts to
> ensure that the write request sent by user space does not extend beyond
> the MTD device's size.  However, that check contains an addition of two
> struct mtd_write_req fields, 'start' and 'len', both of which are u64
> variables.  The result of that addition can overflow, allowing the
> safety check to be bypassed.
> 
> The arguably simplest fix - changing the data types of the relevant
> struct mtd_write_req fields - is not feasible as it would break user
> space.
> 
> Fix by making mtdchar_write_ioctl() truncate the value provided by user
> space in the 'len' field of struct mtd_write_req, so that only the lower
> 32 bits of that field are used, preventing the overflow.
> 
> While the 'ooblen' field of struct mtd_write_req is not currently used
> in any similarly flawed safety check, also truncate it to 32 bits, for
> consistency with the 'len' field and with other MTD routines handling
> OOB data.
> 
> Update include/uapi/mtd/mtd-abi.h accordingly.
> 
> Suggested-by: Richard Weinberger <richard@nod.at>
> Signed-off-by: Michał Kępień <kernel@kempniu.pl>
> ---
> drivers/mtd/mtdchar.c      | 3 +++
> include/uapi/mtd/mtd-abi.h | 4 ++--
> 2 files changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/mtd/mtdchar.c b/drivers/mtd/mtdchar.c
> index d0f9c4b0285c..b2700f8467ff 100644
> --- a/drivers/mtd/mtdchar.c
> +++ b/drivers/mtd/mtdchar.c
> @@ -615,6 +615,9 @@ static int mtdchar_write_ioctl(struct mtd_info *mtd,
> 	if (!usr_oob)
> 		req.ooblen = 0;
> 
> +	req.len &= 0xffffffff;
> +	req.ooblen &= 0xffffffff;
> +

Yeah, I think it is reasonable to limit write requests to 4GiB.

Thanks,
//richard

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