[hackers] [st] base64dec: don't read out of bounds || Avi Halachmi (:avih)

[git]

commit ea4d933ed9d8ce16699c84892a29e070c70b2eb9
Author:     Avi Halachmi (:avih) <avih...@yahoo.com>
AuthorDate: Wed Oct 16 11:17:23 2019 +0300
Commit:     Hiltjo Posthuma <hil...@codemadness.org>
CommitDate: Sun Nov 10 22:45:54 2019 +0100

    base64dec: don't read out of bounds
    
    Previously, base64dec checked terminating input '\0' every 4 calls to
    base64dec_getc, where the latter progressed one or more chars on each
    call, and could read past '\0' in the way it was used.
    
    The input to base64dec currently comes only from OSC 52 escape seq
    (copy to clipboard), and reading past '\0' or even past the buffer
    boundary was easy to trigger.
    
    Also, even if we could trust external input to be valid base64, there
    are different base64 standards, and not all of them require padding
    to 4 bytes blocks (using trailing '=' chars).
    
    It didn't affect short OSC 52 strings because the buffer is initialized
    to 0's, so typically it did stop within the buffer, but if the string
    was trimmed to fit (the buffer is 512 bytes) then it did also read past
    the end of the buffer, and the decoded suffix ended up arbitrary.
    
    This patch makes base64dec_getc not progress past '\0', and instead
    produce fake trailing padding of '='.
    
    Additionally, at base64dec, if padding is detected at the first or
    second byte of a quartet, then we identify it as invalid and abort
    (a valid quartet has at least two leading non-padding bytes).

diff --git a/st.c b/st.c
index ede7ae6..a8f8232 100644
--- a/st.c
+++ b/st.c
@@ -366,7 +366,7 @@ char
 base64dec_getc(const char **src)
 {
        while (**src && !isprint(**src)) (*src)++;
-       return *((*src)++);
+       return **src ? *((*src)++) : '=';  /* emulate padding if string ends */
 }
 
 char *
@@ -384,6 +384,10 @@ base64dec(const char *src)
                int c = base64_digits[(unsigned char) base64dec_getc(&src)];
                int d = base64_digits[(unsigned char) base64dec_getc(&src)];
 
+               /* invalid input. 'a' can be -1, e.g. if src is "\n" (c-str) */
+               if (a == -1 || b == -1)
+                       break;
+
                *dst++ = (a << 2) | ((b & 0x30) >> 4);
                if (c == -1)
                        break;