Re: [PATCH v3 2/2] bootconfig: Fix integer overflow in initrd size check

[Google]

On Thu, 10 Sep 2026 03:05:12 -0700
Breno Leitao <leitao@xxxxxxxxxx> wrote:

> On Thu, Sep 10, 2026 at 10:59:35AM +0900, Masami Hiramatsu (Google) wrote:
> > From: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> > 
> > Sashiko reported that in get_boot_config_from_initrd(), a crafted initrd
> > with a huge bootconfig size (such as 0xFFFFFFFF) can cause the pointer
> > arithmetic:
> > 
> >     data = ((void *)hdr) - size;
> > 
> > to wrap around on 32-bit systems (or when pointer subtraction overflows).
> > Because data wraps around, the subsequent bounds check:
> > 
> >     if ((unsigned long)data < initrd_start)
> > 
> > evaluates to false, bypassing the check. The kernel then calls
> > xbc_calc_checksum(data, size), which attempts to read 4GB of memory,
> > hitting unmapped pages and triggering a fatal kernel page fault during
> > early boot. Furthermore, on 64-bit systems with an initrd > 4.29 GB, an
> > unbounded 32-bit size can similarly bypass the initrd_start check.
> > 
> > Fix this by:
> > 1. Ensuring the initrd is at least large enough to contain the bootconfig
> >    footer and verifying hdr is within the initrd bounds.
> > 2. Checking that size does not exceed XBC_DATA_MAX and does not exceed
> >    the available space between initrd_start and hdr before performing
> >    pointer subtraction.
> > 
> > Fixes: de462e5f1071 ("bootconfig: Fix to remove bootconfig data from initrd while boot")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Reported-by: Sashiko <sashiko-bot@xxxxxxxxxx>
> > Closes: https://lore.kernel.org/all/20260910010137.EE0431F000FF@xxxxxxxxxxxxxxx/
> > Assisted-by: Antigravity:gemini-3.8-flash
> > Signed-off-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> 
> Reviewed-by: Breno Leitao <leitao@xxxxxxxxxx>


Thanks! I'll make a small fix and send v4 with your reviewed-by.

-- 
Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>