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

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

[Masami Hiramatsu (Google)]

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

Sashiko reported that on 32-bit systems, if the size in the bootconfig
footer is corrupted such that adding BOOTCONFIG_FOOTER_SIZE wraps
around (for instance, if size is 0xFFFFFFFF), the size check in
load_xbc_from_initrd() can be bypassed:

    if (stat.st_size < size + BOOTCONFIG_FOOTER_SIZE) {
        pr_err("bootconfig size is too big\n");
        return -E2BIG;
    }

This would lead to load_xbc_fd() being called with a negative size,
resulting in an undersized allocation, heap overflow on read, and
out-of-bounds write on the terminating null byte.

Avoid the integer overflow by checking whether size is greater than
stat.st_size - BOOTCONFIG_FOOTER_SIZE. Since stat.st_size is already
verified to be at least BOOTCONFIG_FOOTER_SIZE earlier in the function,
this subtraction will never underflow.

Fixes: 950313ebf79c ("tools: bootconfig: Add bootconfig command")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/all/20260909161113.16C691F00A3A@smtp.kernel.org/
Assisted-by: Antigravity:gemini-3.8-flash
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
---
 tools/bootconfig/main.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/bootconfig/main.c b/tools/bootconfig/main.c
index 7dc9fff9b637..b380ad6777fa 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) {
 		pr_err("bootconfig size is too big\n");
 		return -E2BIG;
 	}

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

[Masami Hiramatsu]

On Thu, 10 Sep 2026 09:30:46 +0900
"Masami Hiramatsu (Google)" <mhiramat@kernel.org> wrote:

> From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> 
> Sashiko reported that on 32-bit systems, if the size in the bootconfig
> footer is corrupted such that adding BOOTCONFIG_FOOTER_SIZE wraps
> around (for instance, if size is 0xFFFFFFFF), the size check in
> load_xbc_from_initrd() can be bypassed:
> 
>     if (stat.st_size < size + BOOTCONFIG_FOOTER_SIZE) {
>         pr_err("bootconfig size is too big\n");
>         return -E2BIG;
>     }
> 
> This would lead to load_xbc_fd() being called with a negative size,
> resulting in an undersized allocation, heap overflow on read, and
> out-of-bounds write on the terminating null byte.
> 
> Avoid the integer overflow by checking whether size is greater than
> stat.st_size - BOOTCONFIG_FOOTER_SIZE. Since stat.st_size is already
> verified to be at least BOOTCONFIG_FOOTER_SIZE earlier in the function,
> this subtraction will never underflow.
> 
> Fixes: 950313ebf79c ("tools: bootconfig: Add bootconfig command")
> Reported-by: Sashiko <sashiko-bot@kernel.org>
> Closes: https://lore.kernel.org/all/20260909161113.16C691F00A3A@smtp.kernel.org/
> Assisted-by: Antigravity:gemini-3.8-flash

OOps, I forgot to add:

Cc: stable@vger.kernel.org

Thanks,

> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> ---
>  tools/bootconfig/main.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/tools/bootconfig/main.c b/tools/bootconfig/main.c
> index 7dc9fff9b637..b380ad6777fa 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) {
>  		pr_err("bootconfig size is too big\n");
>  		return -E2BIG;
>  	}
> 


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

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

[sashiko-bot]

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.

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;
>  	}

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/178900024679.183840.10345859264677533824.stgit@devnote2?part=1

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

[Masami Hiramatsu]

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>