[PATCH v6 0/2] lib/vsprintf: Fixes size check

[PATCH v6 0/2] lib/vsprintf: Fixes size check

[Masami Hiramatsu (Google)]

Hi,

Here is the 6th version of patches to fix vsnprintf().

 - Fix to limit the size of width and precision.
 - Warn if the return size is over INT_MAX.

Previous version is here;

https://lore.kernel.org/all/177444525139.185641.12184379647176430297.stgit@devnote2/

This version just makes function header and WARN_ONCE() to one line[1/2].
No functional change.

Thank you,

---

Masami Hiramatsu (Google) (2):
      lib/vsprintf: Fix to check field_width and precision
      lib/vsprintf: Limit the returning size to INT_MAX


 lib/vsprintf.c |   51 +++++++++++++++++++++++++++------------------------
 1 file changed, 27 insertions(+), 24 deletions(-)

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

[PATCH v6 1/2] lib/vsprintf: Fix to check field_width and precision

[Masami Hiramatsu (Google)]

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

Check the field_width and presition correctly. Previously it depends
on the bitfield conversion from int to check out-of-range error.
However, commit 938df695e98d ("vsprintf: associate the format state
with the format pointer") changed those fields to int.
We need to check the out-of-range correctly without bitfield
conversion.

Fixes: 938df695e98d ("vsprintf: associate the format state with the format pointer")
Reported-by: David Laight <david.laight.linux@gmail.com>
Closes: https://lore.kernel.org/all/20260318151250.40fef0ab@pumpkin/
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
---
 Changes in v6:
  - Make the function header of set_field_width() and set_precision(), and
   WARN_ONCE()  to be one line.
 Changes in v5:
  - Drop negative precision support.
  - The field and precision passed as string literals are also checked.
 Changes in v4:
  - Do clamp() first.
  - Accept negative precision (this means no precision) .
  - Change the warning message for width.
 Changes in v3:
  - Check and update width and precision before assigning to spec.
 Changes in v2:
  - Fix to use logical split.
---
 lib/vsprintf.c |   36 ++++++++++++++----------------------
 1 file changed, 14 insertions(+), 22 deletions(-)

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 800b8ac49f53..10a1b21c8920 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -2640,6 +2640,18 @@ static unsigned char spec_flag(unsigned char c)
 	return (c < sizeof(spec_flag_array)) ? spec_flag_array[c] : 0;
 }
 
+static void set_field_width(struct printf_spec *spec, int width)
+{
+	spec->field_width = clamp(width, -FIELD_WIDTH_MAX, FIELD_WIDTH_MAX);
+	WARN_ONCE(spec->field_width != width, "field width %d out of range", width);
+}
+
+static void set_precision(struct printf_spec *spec, int prec)
+{
+	spec->precision = clamp(prec, 0, PRECISION_MAX);
+	WARN_ONCE(spec->precision < prec, "precision %d too large", prec);
+}
+
 /*
  * Helper function to decode printf style format.
  * Each call decode a token from the format and return the
@@ -2710,7 +2722,7 @@ struct fmt format_decode(struct fmt fmt, struct printf_spec *spec)
 	spec->field_width = -1;
 
 	if (isdigit(*fmt.str))
-		spec->field_width = skip_atoi(&fmt.str);
+		set_field_width(spec, skip_atoi(&fmt.str));
 	else if (unlikely(*fmt.str == '*')) {
 		/* it's the next argument */
 		fmt.state = FORMAT_STATE_WIDTH;
@@ -2724,9 +2736,7 @@ struct fmt format_decode(struct fmt fmt, struct printf_spec *spec)
 	if (unlikely(*fmt.str == '.')) {
 		fmt.str++;
 		if (isdigit(*fmt.str)) {
-			spec->precision = skip_atoi(&fmt.str);
-			if (spec->precision < 0)
-				spec->precision = 0;
+			set_precision(spec, skip_atoi(&fmt.str));
 		} else if (*fmt.str == '*') {
 			/* it's the next argument */
 			fmt.state = FORMAT_STATE_PRECISION;
@@ -2799,24 +2809,6 @@ struct fmt format_decode(struct fmt fmt, struct printf_spec *spec)
 	return fmt;
 }
 
-static void
-set_field_width(struct printf_spec *spec, int width)
-{
-	spec->field_width = width;
-	if (WARN_ONCE(spec->field_width != width, "field width %d too large", width)) {
-		spec->field_width = clamp(width, -FIELD_WIDTH_MAX, FIELD_WIDTH_MAX);
-	}
-}
-
-static void
-set_precision(struct printf_spec *spec, int prec)
-{
-	spec->precision = prec;
-	if (WARN_ONCE(spec->precision != prec, "precision %d too large", prec)) {
-		spec->precision = clamp(prec, 0, PRECISION_MAX);
-	}
-}
-
 /*
  * Turn a 1/2/4-byte value into a 64-bit one for printing: truncate
  * as necessary and deal with signedness.

[PATCH v6 2/2] lib/vsprintf: Limit the returning size to INT_MAX

[Masami Hiramatsu (Google)]

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

The return value of vsnprintf() and bstr_printf() can overflow INT_MAX
and return a minus value. In the @size is checked input overflow, but
it does not check the output, which is expected required size.

This should never happen but it should be checked and limited.

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
---
 Changes in v5:
  - Fix bstr_printf() too.
 Changes in v4:
  - Add Petr's reviewed-by. (Thanks!)
 Changes in v3:
  - Use local variable for better readability.
---
 lib/vsprintf.c |   15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 10a1b21c8920..d0aa6b34f0e4 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -2856,6 +2856,7 @@ static unsigned long long convert_num_spec(unsigned int val, int size, struct pr
 int vsnprintf(char *buf, size_t size, const char *fmt_str, va_list args)
 {
 	char *str, *end;
+	size_t ret_size;
 	struct printf_spec spec = {0};
 	struct fmt fmt = {
 		.str = fmt_str,
@@ -2975,8 +2976,12 @@ int vsnprintf(char *buf, size_t size, const char *fmt_str, va_list args)
 	}
 
 	/* the trailing null byte doesn't count towards the total */
-	return str-buf;
+	ret_size = str - buf;
 
+	/* Make sure the return value is within the positive integer range */
+	if (WARN_ON_ONCE(ret_size > INT_MAX))
+		ret_size = INT_MAX;
+	return ret_size;
 }
 EXPORT_SYMBOL(vsnprintf);
 
@@ -3280,6 +3285,7 @@ int bstr_printf(char *buf, size_t size, const char *fmt_str, const u32 *bin_buf)
 	struct printf_spec spec = {0};
 	char *str, *end;
 	const char *args = (const char *)bin_buf;
+	size_t ret_size;
 
 	if (WARN_ON_ONCE(size > INT_MAX))
 		return 0;
@@ -3428,7 +3434,12 @@ int bstr_printf(char *buf, size_t size, const char *fmt_str, const u32 *bin_buf)
 #undef get_arg
 
 	/* the trailing null byte doesn't count towards the total */
-	return str - buf;
+	ret_size =  str - buf;
+
+	/* Make sure the return value is within the positive integer range */
+	if (WARN_ON_ONCE(ret_size > INT_MAX))
+		ret_size = INT_MAX;
+	return ret_size;
 }
 EXPORT_SYMBOL_GPL(bstr_printf);
 

Re: [PATCH v6 0/2] lib/vsprintf: Fixes size check

[Petr Mladek]

On Thu 2026-03-26 21:11:50, Masami Hiramatsu (Google) wrote:
> Hi,
> 
> Here is the 6th version of patches to fix vsnprintf().
> 
>  - Fix to limit the size of width and precision.
>  - Warn if the return size is over INT_MAX.
> 
> Previous version is here;
> 
> https://lore.kernel.org/all/177444525139.185641.12184379647176430297.stgit@devnote2/
> 
> This version just makes function header and WARN_ONCE() to one line[1/2].
> No functional change.
> 
> Thank you,
> 
> ---
> 
> Masami Hiramatsu (Google) (2):
>       lib/vsprintf: Fix to check field_width and precision
>       lib/vsprintf: Limit the returning size to INT_MAX

I have finally got back to this series. The last version looks
good to me. It is very conservative. For both patches:

Reviewed-by: Petr Mladek <pmladek@suse.com>

I am going to queue it via the printk tree...

Best Regards,
Petr

PS: My understanding is that it should be safe to make the precision
    handling compatible with glibc/POSIX printf() and treat negative
    values as no given values. Feel free to send this change as
    a followup fix.

Re: [PATCH v6 0/2] lib/vsprintf: Fixes size check

[Petr Mladek]

On Thu 2026-03-26 21:11:50, Masami Hiramatsu (Google) wrote:
> Hi,
> 
> Here is the 6th version of patches to fix vsnprintf().
> 
>  - Fix to limit the size of width and precision.
>  - Warn if the return size is over INT_MAX.
> 
> Previous version is here;
> 
> https://lore.kernel.org/all/177444525139.185641.12184379647176430297.stgit@devnote2/
> 
> This version just makes function header and WARN_ONCE() to one line[1/2].
> No functional change.
> 
> Thank you,
> 
> ---
> 
> Masami Hiramatsu (Google) (2):
>       lib/vsprintf: Fix to check field_width and precision
>       lib/vsprintf: Limit the returning size to INT_MAX

JFYI, the patchset has been commited into printk/linux.git,
branch for-7.2-vsprintf-size-checks.

Best Regards,
Petr