tunables: Terminate if end of input is reached (CVE-2023-4911)

The string parsing routine may end up writing beyond bounds of tunestr
if the input tunable string is malformed, of the form name=name=val.
This gets processed twice, first as name=name=val and next as name=val,
resulting in tunestr being name=name=val:name=val, thus overflowing
tunestr.

Terminate the parsing loop at the first instance itself so that tunestr
does not overflow.

This also fixes up tst-env-setuid-tunables to actually handle failures
correct and add new tests to validate the fix for this CVE.

Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>

1 files changed, 10 insertions, 7 deletions

diff --git a/elf/dl-tunables.c b/elf/dl-tunables.c
index 62b7332d95..cae67efa0a 100644
--- a/elf/dl-tunables.c
+++ b/elf/dl-tunables.c
@@ -180,11 +180,7 @@ parse_tunables (char *tunestr, char *valstring)
       /* If we reach the end of the string before getting a valid name-value
 	 pair, bail out.  */
       if (p[len] == '\0')
-	{
-	  if (__libc_enable_secure)
-	    tunestr[off] = '\0';
-	  return;
-	}
+	break;
 
       /* We did not find a valid name-value pair before encountering the
 	 colon.  */
@@ -244,9 +240,16 @@ parse_tunables (char *tunestr, char *valstring)
 	    }
 	}
 
-      if (p[len] != '\0')
-	p += len + 1;
+      /* We reached the end while processing the tunable string.  */
+      if (p[len] == '\0')
+	break;
+
+      p += len + 1;
     }
+
+  /* Terminate tunestr before we leave.  */
+  if (__libc_enable_secure)
+    tunestr[off] = '\0';
 }
 
 /* Enable the glibc.malloc.check tunable in SETUID/SETGID programs only when