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From: Miquel Raynal <miquel.raynal@bootlin.com>
To: Daniel Golle <daniel@makrotopia.org>
Cc: Richard Weinberger <richard@nod.at>,
	 Vignesh Raghavendra <vigneshr@ti.com>,
	 Boris Brezillon <bbrezillon@kernel.org>,
	linux-mtd@lists.infradead.org,  linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mtd: nand: bbt: clamp GENMASK high bit to word boundary
Date: Tue, 21 Apr 2026 09:32:02 +0200	[thread overview]
Message-ID: <87pl3s7or1.fsf@bootlin.com> (raw)
In-Reply-To: <adzL_tlZTntTVPD-@makrotopia.org> (Daniel Golle's message of "Mon, 13 Apr 2026 11:57:02 +0100")

Hi Daniel,

>> > When a BBT entry straddles an unsigned long boundary, the GENMASK in
>> > nanddev_bbt_set_block_status() can potentially overflow because
>> > offs + bits_per_block - 1 can theoretically exceed BITS_PER_LONG - 1.
>> > Clamp the high bit so only bits within the current word are masked.
>> > The cross-word portion is already handled by the pos[1] block below.
>> >
>> > Discovered by UBSAN: shift-out-of-bounds in
>> > drivers/mtd/nand/bbt.c:116:13
>> > shift exponent 18446744073709551614 is too large for 64-bit type
>> > 'long unsigned int'
>> 
>> How likely is that? It doesn't matter how many bits you use per blocks
>> (today is 2), it would require a NAND chip that covers an entire country
>> to reach that number of blocks. If an attacker plays with that value,
>> does it really matter? Apart from writing out of bounds -which is
>> physically impossible, we are not talking about virtual memory here- and
>> get an error later on, I do not see a good reason for this.
>> 
>> Honestly, I find the final result much less readable than before for no
>> obvious added value IMO. But maybe I am looking at this the wrong way?
>
> It's just the only UBSAN warning I get to see on a recent kernel and my
> primary goal here was to make the warning go away. Adding an assertion
> to ensure 'offs' is clamped to will likely also make the warning go
> away.

I believe that's a more appropriate approach, if you don't mind.

Thanks,
Miquèl

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      reply	other threads:[~2026-04-21  7:32 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2026-04-12  0:05 [PATCH] mtd: nand: bbt: clamp GENMASK high bit to word boundary Daniel Golle
2026-04-13  8:12 ` Miquel Raynal
2026-04-13 10:57   ` Daniel Golle
2026-04-21  7:32     ` Miquel Raynal [this message]

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