[SLOF] [PATCH v2 3/3] lib/libnet/pxelinux: Fix two off-by-one bugs in the pxelinux.cfg parser

Thomas Huth thuth at redhat.com
Tue Jun 5 20:11:49 AEST 2018

There are two small bugs in the pxelinux.cfg parser:

1. If the file does not end with a '\n', the code set 'eol = cfg + cfgsize'
and later wrote a NUL character to *eol, i.e. it wrote the NUL character
beyond the end of the buffer. We've got to use 'eol = cfg + cfgsize - 1'

2. The code always replaced the last byte of the buffer with a NUL character
to get a proper termination. If the config file ends with a required character
(e.g. the last line is a KERNEL or INITRD line and the file does not have
a '\n' at the end), the last character got lost. Move the obligation for the
terminating NUL character to the caller instead so that we can be sure to
have a proper terminated buffer in pxelinux_parse_cfg() without the need to
blindly overwrite the last character here.

Reviewed-by: Greg Kurz <groug at kaod.org>
Signed-off-by: Thomas Huth <thuth at redhat.com>
 lib/libnet/pxelinux.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/lib/libnet/pxelinux.c b/lib/libnet/pxelinux.c
index 939a92c..c4ac5d5 100644
--- a/lib/libnet/pxelinux.c
+++ b/lib/libnet/pxelinux.c
@@ -151,8 +151,9 @@ static int pxelinux_load_cfg(filename_ip_t *fn_ip, uint8_t *mac, const char *uui
  * in entries point to the original location in the cfg buffer area. The cfg
  * buffer is altered for this, too, e.g. terminating NUL-characters are put
  * into the right locations.
- * @param cfg          Pointer to the buffer with contents of the config file
- * @param cfgsize      Size of the cfg buffer
+ * @param cfg          Pointer to the buffer with contents of the config file.
+ *                     The caller must make sure that it is NUL-terminated.
+ * @param cfgsize      Size of the cfg data (including the terminating NUL)
  * @param entries      Pointer to array where the results should be put into
  * @param max_entries  Number of available slots in the entries array
  * @param def_ent      Used to return the index of the default entry
@@ -167,12 +168,10 @@ int pxelinux_parse_cfg(char *cfg, int cfgsize, struct pl_cfg_entry *entries,
 	*def_ent = 0;
-	cfg[cfgsize - 1] = 0;  /* Make sure it is NUL-terminated */
 	while (ptr < cfg + cfgsize && num_entries < max_entries) {
 		eol = strchr(ptr, '\n');
 		if (!eol) {
-			eol = cfg + cfgsize;
+			eol = cfg + cfgsize - 1;
 		nextptr = eol + 1;
 		do {
@@ -246,5 +245,7 @@ int pxelinux_load_parse_cfg(filename_ip_t *fn_ip, uint8_t *mac, const char *uuid
 		return rc;
 	assert(rc < cfgsize);
+	cfgbuf[rc++] = '\0';	/* Make sure it is NUL-terminated */
 	return pxelinux_parse_cfg(cfgbuf, rc, entries, max_entries, def_ent);

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