diff mbox

opal: Fix buffer overrun in print_* functions.

Message ID 20150112192412.12078.15335.stgit@mars
State Superseded
Headers show

Commit Message

Mahesh J Salgaonkar Jan. 12, 2015, 7:25 p.m. UTC 
From: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>

While running HMI tests I saw massive corruption in OPAL for one of the
HMI test that injects TB error. On investigation I found that
vsnprintf()->print_itoa() was the culprit. print_itoa function uses tmp
array of size 16 to convert unsigned long value to ASCII. But an unsigned
value of 0xffffffffffffffff needs atleast 25 characters to print its ASCII
representation. This caused an array to overflow resulting into corruption,
unpredictable behavior and finally system termination.

We could fix this by increasing the size of tmp[] array but that still won't
fix this bug completely. While auditing vsnprintf() function I found that
it makes use of print_*() functions to write data to buffer. But none of
the print_* function have any check on buffer size before writing data to it.
Without size check print_*() functions can easily overrun buffer passed by
vprlog()->vsnprintf()->print_format()->print_*() causing massive corruption
leading to unpredictable behavior.

This patch fixes this bug by modifying print_*() function to check for
buffer size to avoid buffer overrun.

- Modified print_format(), print_fill() and print_itoa() to take bufsize
  as argument and added a size check while writing to buffer.
- Remove temporary array from print_itoa(), instead write data directly to
  the buffer.
- Added two new function print_str_fill() and print_str() to be used as
  helper routine for '%s' fmt case in print_format() function. These new
  routines now has a check on buffer size while copying NULL terminated
  string to buffer.
- Added "!bufsize" check in vsnprintf() to avoid buffer overrun while
  setting NULL character before returning.

I ran HMI tests with this patch successfully. I also tested this patch by
reducing size of the buffer (in core/console-log.c:vprlog()) from 320 to 50
and booted the lid successfully with no corruption at all.

Please review this patch.

Signed-off-by: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
---
 libc/stdio/vsnprintf.c |  163 ++++++++++++++++++++++++++++++++----------------
 1 file changed, 108 insertions(+), 55 deletions(-)

Comments

Stewart Smith Jan. 14, 2015, 12:42 a.m. UTC | #1 
Mahesh J Salgaonkar <mahesh@linux.vnet.ibm.com> writes:
> From: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
>
> While running HMI tests I saw massive corruption in OPAL for one of the
> HMI test that injects TB error. On investigation I found that
> vsnprintf()->print_itoa() was the culprit. print_itoa function uses tmp
> array of size 16 to convert unsigned long value to ASCII. But an unsigned
> value of 0xffffffffffffffff needs atleast 25 characters to print its ASCII
> representation. This caused an array to overflow resulting into corruption,
> unpredictable behavior and finally system termination.

This looks like fun and totally requires some close review.

I'd love it if you could add some unit tests for it that showed both the
bugs and that the fixes work.
Benjamin Herrenschmidt Jan. 14, 2015, 8:56 a.m. UTC | #2 
On Wed, 2015-01-14 at 13:42 +1300, Stewart Smith wrote:
> Mahesh J Salgaonkar <mahesh@linux.vnet.ibm.com> writes:
> > From: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
> >
> > While running HMI tests I saw massive corruption in OPAL for one of the
> > HMI test that injects TB error. On investigation I found that
> > vsnprintf()->print_itoa() was the culprit. print_itoa function uses tmp
> > array of size 16 to convert unsigned long value to ASCII. But an unsigned
> > value of 0xffffffffffffffff needs atleast 25 characters to print its ASCII
> > representation. This caused an array to overflow resulting into corruption,
> > unpredictable behavior and finally system termination.
> 
> This looks like fun and totally requires some close review.
> 
> I'd love it if you could add some unit tests for it that showed both the
> bugs and that the fixes work.

Interesting that -fstack-protector didn't catch it ...

Ben.
diff mbox

Patch

diff --git a/libc/stdio/vsnprintf.c b/libc/stdio/vsnprintf.c
index b2f0b94..2355f8b 100644
--- a/libc/stdio/vsnprintf.c
+++ b/libc/stdio/vsnprintf.c
@@ -22,59 +22,99 @@  static const unsigned long long convert[] = {
 	0xFFFFFFFFFFULL, 0xFFFFFFFFFFFFULL, 0xFFFFFFFFFFFFFFULL, 0xFFFFFFFFFFFFFFFFULL
 };
 
+static int
+print_str_fill(char **buffer, size_t bufsize, char *sizec,
+					const char *str, char c)
+{
+	int i, sizei, len;
+	char *bstart = *buffer;
 
+	sizei = strtoul(sizec, NULL, 10);
+	len = strlen(str);
+	if (sizei > len) {
+		for (i = 0;
+			(i < (sizei - len)) && ((*buffer - bstart) < bufsize);
+									i++) {
+			**buffer = c;
+			*buffer += 1;
+		}
+	}
+	return 1;
+}
 
 static int
-print_itoa(char **buffer, unsigned long value, unsigned short base, bool upper)
+print_str(char **buffer, size_t bufsize, const char *str)
+{
+	char *bstart = *buffer;
+	int i;
+
+	for (i = 0; (i < strlen(str)) && ((*buffer - bstart) < bufsize); i++) {
+		**buffer = str[i];
+		*buffer += 1;
+	}
+	return 1;
+}
+
+static unsigned int __attrconst
+print_intlen(unsigned long value, unsigned short int base)
+{
+	int i = 0;
+
+	while (value > 0) {
+		value /= base;
+		i++;
+	}
+	if (i == 0)
+		i = 1;
+	return i;
+}
+
+static int
+print_itoa(char **buffer, size_t bufsize, unsigned long value,
+					unsigned short base, bool upper)
 {
 	const char zeichen[] = {'0','1','2','3','4','5','6','7','8','9','a','b','c','d','e','f'};
 	char c;
-	int i = 0;
-	char tmp[16];
+	int i, len;
 
 	if(base <= 2 || base > 16)
 		return 0;
 
+	len = i = print_intlen(value, base);
+
+	/* Don't print to buffer if bufsize is not enough. */
+	if (len > bufsize)
+		return 0;
+
 	do {
 		c = zeichen[value % base];
 		if (upper)
 			c = toupper(c);
-		tmp[i++] = c;
+
+		(*buffer)[--i] = c;
 		value /= base;
 	} while(value);
 
-	while (i--) {
-		**buffer = tmp[i];
-		*buffer += 1;
-	}
+	*buffer += len;
 
 	return 1;
 }
 
 
-static unsigned int __attrconst
-print_intlen(unsigned long value, unsigned short int base)
-{
-	int i = 0;
-
-	while(value > 0) {
-		value /= base;
-		i++;
-	}
-	if(i == 0) i = 1;
-	return i;
-}
-
 
 static int
-print_fill(char **buffer, char *sizec, unsigned long size, unsigned short int base, char c, int optlen)
+print_fill(char **buffer, size_t bufsize, char *sizec, unsigned long size,
+				unsigned short int base, char c, int optlen)
 {
 	int i, sizei, len;
+	char *bstart = *buffer;
 
 	sizei = strtoul(sizec, NULL, 10);
  	len = print_intlen(size, base) + optlen;
-	if(sizei > len) {
-		for(i = 0; i < (sizei - len); i++) {
+	if (sizei > len) {
+		for (i = 0;
+			(i < (sizei - len)) && ((*buffer - bstart) < bufsize);
+									i++) {
 			**buffer = c;
 			*buffer += 1;
 		}
@@ -85,10 +125,10 @@  print_fill(char **buffer, char *sizec, unsigned long size, unsigned short int ba
 
 
 static int
-print_format(char **buffer, const char *format, void *var)
+print_format(char **buffer, size_t bufsize, const char *format, void *var)
 {
-	unsigned long start;
-	unsigned int i = 0, sizei = 0, len = 0, length_mod = sizeof(int);
+	char *start;
+	unsigned int i = 0, length_mod = sizeof(int);
 	unsigned long value = 0;
 	unsigned long signBit;
 	char *form, sizec[32];
@@ -96,7 +136,7 @@  print_format(char **buffer, const char *format, void *var)
 	bool upper = false;
 
 	form  = (char *) format;
-	start = (unsigned long) *buffer;
+	start = *buffer;
 
 	form++;
 	if(*form == '0' || *form == '.') {
@@ -104,7 +144,7 @@  print_format(char **buffer, const char *format, void *var)
 		form++;
 	}
 
-	while(*form != '\0') {
+	while ((*form != '\0') && ((*buffer - start) < bufsize)) {
 		switch(*form) {
 			case 'u':
 			case 'd':
@@ -117,53 +157,54 @@  print_format(char **buffer, const char *format, void *var)
 					*buffer += 1;
 					value = (-(unsigned long)value) & convert[length_mod];
 				}
-				print_fill(buffer, sizec, value, 10, sign, 0);
-				print_itoa(buffer, value, 10, upper);
+				print_fill(buffer, bufsize - (*buffer - start),
+						sizec, value, 10, sign, 0);
+				print_itoa(buffer, bufsize - (*buffer - start),
+							value, 10, upper);
 				break;
 			case 'X':
 				upper = true;
 			case 'x':
 				sizec[i] = '\0';
 				value = (unsigned long) var & convert[length_mod];
-				print_fill(buffer, sizec, value, 16, sign, 0);
-				print_itoa(buffer, value, 16, upper);
+				print_fill(buffer, bufsize - (*buffer - start),
+						sizec, value, 16, sign, 0);
+				print_itoa(buffer, bufsize - (*buffer - start),
+							value, 16, upper);
 				break;
 			case 'O':
 			case 'o':
 				sizec[i] = '\0';
 				value = (long int) var & convert[length_mod];
-				print_fill(buffer, sizec, value, 8, sign, 0);
-				print_itoa(buffer, value, 8, upper);
+				print_fill(buffer, bufsize - (*buffer - start),
+						sizec, value, 8, sign, 0);
+				print_itoa(buffer, bufsize - (*buffer - start),
+							value, 8, upper);
 				break;
 			case 'p':
 				sizec[i] = '\0';
-				print_fill(buffer, sizec, (unsigned long) var, 16, ' ', 2);
-				**buffer = '0';
-				*buffer += 1;	
-				**buffer = 'x';
-				*buffer += 1;
-				print_itoa(buffer,(unsigned long) var, 16, upper);
+				print_fill(buffer, bufsize - (*buffer - start),
+					sizec, (unsigned long) var, 16, ' ', 2);
+				print_str(buffer, bufsize - (*buffer - start),
+									"0x");
+				print_itoa(buffer, bufsize - (*buffer - start),
+						(unsigned long) var, 16, upper);
 				break;
 			case 'c':
 				sizec[i] = '\0';
-				print_fill(buffer, sizec, 1, 10, ' ', 0);
+				print_fill(buffer, bufsize - (*buffer - start),
+							sizec, 1, 10, ' ', 0);
 				**buffer = (unsigned long) var;
 				*buffer += 1;
 				break;
 			case 's':
 				sizec[i] = '\0';
-				sizei = strtoul(sizec, NULL, 10);
-				len = strlen((char *) var);
-				if(sizei > len) {
-					for(i = 0; i < (sizei - len); i++) {
-						**buffer = ' ';
-						*buffer += 1;
-					}
-				}
-				for(i = 0; i < strlen((char *) var); i++) {
-					**buffer = ((char *) var)[i];
-					*buffer += 1;
-				}
+				print_str_fill(buffer,
+					bufsize - (*buffer - start), sizec,
+							(char *) var, ' ');
+
+				print_str(buffer, bufsize - (*buffer - start),
+								(char *) var);
 				break;
 			case 'l':
 				form++;
@@ -210,6 +251,16 @@  vsnprintf(char *buffer, size_t bufsize, const char *format, va_list arg)
 	bstart = buffer;
 	ptr = (char *) format;
 
+	/*
+	 * Return from here if size passed is zero, otherwise we would
+	 * overrun buffer while setting NULL character at the end.
+	 */
+	if (!bufsize)
+		return 0;
+
+	/* Leave one space for NULL character */
+	bufsize--;
+
 	while(*ptr != '\0' && (buffer - bstart) < bufsize)
 	{
 		if(*ptr == '%') {
@@ -228,7 +279,9 @@  vsnprintf(char *buffer, size_t bufsize, const char *format, va_list arg)
 			if(*ptr == '%') {
 				*buffer++ = '%';
 			} else {
-				print_format(&buffer, formstr, va_arg(arg, void *));
+				print_format(&buffer,
+					bufsize - (buffer - bstart),
+					formstr, va_arg(arg, void *));
 			}
 			ptr++;
 		} else {