Re: [PATCH] tools/bootconfig: Fix integer overflow in size check

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

On Thu, 10 Sep 2026 00:40:55 +0000
sashiko-bot@kernel.org wrote:

> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> 
> Pre-existing issues:
> - [High] Incomplete Fix / Integer Truncation Leading to Heap Overflow
> --
> 
> commit 2d027c4ee8625f17122dea0afb65c1e4b21ae531
> Author: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> 
>     tools/bootconfig: Fix integer overflow in size check
>     
>     This commit modifies the size check in load_xbc_from_initrd() to prevent
>     an integer overflow on 32-bit systems. It changes the condition to
>     subtract BOOTCONFIG_FOOTER_SIZE from stat.st_size instead of adding it
>     to the bootconfig size.
> 
>     Closes: https://lore.kernel.org/all/20260909161113.16C691F00A3A@smtp.kernel.org/
> 
> > diff --git a/tools/bootconfig/main.c b/tools/bootconfig/main.c
> > index 7dc9fff9b6370..b380ad6777faa 100644
> > --- a/tools/bootconfig/main.c
> > +++ b/tools/bootconfig/main.c
> > @@ -218,7 +218,7 @@ static int load_xbc_from_initrd(int fd, char **buf)
> >  	csum = le32toh(csum);
> >  
> >  	/* Wrong size error  */
> > -	if (stat.st_size < size + BOOTCONFIG_FOOTER_SIZE) {
> > +	if (size > stat.st_size - BOOTCONFIG_FOOTER_SIZE) {
> 
> [Severity: High]
> This is a pre-existing issue, but the fix doesn't seem to completely resolve
> the integer truncation and heap overflow vulnerabilities.
> 
> Can this size check still be bypassed for large files? Since stat.st_size is a
> 64-bit off_t, if the file size is larger than 4.29 GB, stat.st_size -
> BOOTCONFIG_FOOTER_SIZE can exceed UINT32_MAX. A corrupted uint32_t size of
> 0xFFFFFFFF would bypass this check because it is always smaller than the
> right side.
> 
> If bypassed, does passing 0xFFFFFFFF to load_xbc_fd() cause an integer
> truncation and heap overflow?
> 
> tools/bootconfig/main.c:load_xbc_fd() {
>     ...
>     static int load_xbc_fd(int fd, char **buf, int size)
>     ...
>         *buf = malloc(size + 1);
>         if (!*buf)
>             return -ENOMEM;
>     
>         ret = read(fd, *buf, size);
>     ...
>         (*buf)[size] = '\0';
>     ...
> }
> 
> Because size is an int, 0xFFFFFFFF is truncated to -1. This results in
> malloc(0) which returns a valid heap chunk. The read() call then receives -1
> which gets cast to SIZE_MAX, overflowing the heap chunk. Finally, a null byte
> is written to a negative array index (-1), causing an out-of-bounds write.

This part should be fixed in v2. But below is another problem.

> 
> Does this same vulnerability also affect load_xbc_file() when processing
> standalone files >= 2GB?
> 
> tools/bootconfig/main.c:load_xbc_file() {
>     ...
>     ret = fstat(fd, &stat);
>     ...
>     ret = load_xbc_fd(fd, buf, stat.st_size);
>     ...
> }
> 
> This implicitly truncates the 64-bit off_t stat.st_size into the 32-bit int
> size parameter of load_xbc_fd().
> 
> >  		pr_err("bootconfig size is too big\n");
> >  		return -E2BIG;
> >  	}

OK, let me update the patch.

Thank you,

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