Re: [PATCH v4 2/2] bootconfig: Fix integer overflow in initrd size check

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Fri, 11 Sep 2026 01:29:48 +0900
Sang-Heon Jeon <ekffu200098@gmail.com> wrote:

> On Fri, Sep 11, 2026 at 12:15 AM Masami Hiramatsu (Google)
> <mhiramat@kernel.org> wrote:
> >
> > From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> >
> > 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@vger.kernel.org
> > Reported-by: Sashiko <sashiko-bot@kernel.org>
> > Closes: https://lore.kernel.org/all/20260910010137.EE0431F000FF@smtp.kernel.org/
> > Assisted-by: Antigravity:gemini-3.8-flash
> > Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> > ---
> >  Changes in v4:
> >   - Accurate the error message for size > XBC_DATA_MAX in
> >     get_boot_config_from_initrd().
> >   - Remove redundant (and wrong) size >= XBC_DATA_MAX check from
> >     setup_boot_config().
> > ---
> >  init/main.c |   25 +++++++++++++++----------
> >  1 file changed, 15 insertions(+), 10 deletions(-)
> >
> > diff --git a/init/main.c b/init/main.c
> > index 2613d3f9b3ce..16749bb7a219 100644
> > --- a/init/main.c
> > +++ b/init/main.c
> > @@ -277,7 +277,8 @@ static void * __init get_boot_config_from_initrd(size_t *_size)
> >         u8 *hdr;
> >         int i;
> >
> > -       if (!initrd_end)
> > +       if (!initrd_end || initrd_end < initrd_start ||
> > +           initrd_end - initrd_start < BOOTCONFIG_MAGIC_LEN + 8)
> >                 return NULL;
> 
> (nit) how about moving BOOTCONFIG_FOOTER_SIZE from tools and using it instead?

OK, that will be done in cleanup patch.

> 
> >         data = (char *)initrd_end - BOOTCONFIG_MAGIC_LEN;
> > @@ -294,16 +295,26 @@ static void * __init get_boot_config_from_initrd(size_t *_size)
> >
> >  found:
> >         hdr = (u8 *)(data - 8);
> > +       if ((unsigned long)hdr < initrd_start)
> > +               return NULL;
> > +
> >         size = get_unaligned_le32(hdr);
> >         csum = get_unaligned_le32(hdr + 4);
> >
> > -       data = ((void *)hdr) - size;
> > -       if ((unsigned long)data < initrd_start) {
> > -               pr_err("bootconfig size %d is greater than initrd size %ld\n",
> > +       if (size > XBC_DATA_MAX) {
> > +               pr_err("bootconfig size %u is greater than max size %d\n",
> > +                       size, XBC_DATA_MAX);
> > +               return NULL;
> > +       }
> > +
> > +       if (size > ((unsigned long)hdr - initrd_start)) {
> > +               pr_err("bootconfig size %u is greater than initrd size %lu\n",
> >                         size, initrd_end - initrd_start);
> >                 return NULL;
> >         }
> >
> > +       data = ((void *)hdr) - size;
> > +
> >         if (xbc_calc_checksum(data, size) != csum) {
> >                 pr_err("bootconfig checksum failed\n");
> >                 return NULL;
> > @@ -394,12 +405,6 @@ static void __init setup_boot_config(void)
> >                 return;
> >         }
> >
> > -       if (size >= XBC_DATA_MAX) {
> > -               pr_err("bootconfig size %ld greater than max size %d\n",
> > -                       (long)size, XBC_DATA_MAX);
> > -               return;
> > -       }
> > -
> 
> Nice cleanup!
> 
> >         ret = xbc_init(data, size, &msg, &pos);
> >         if (ret < 0) {
> >                 if (pos < 0)
> >
> 
> Thanks for doing this work!
> 
> Reviewed-by: Sang-Heon Jeon <ekffu200098@gmail.com>
> 

Thanks!


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>