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kernel: reject bogus two byte "#!" shell scripts in sysexec()

- reject files smaller or equal to two bytes, they are bogus
- fix out of bounds access in shargs() when n <= 2
- only copy the bytes read into line buffer
- use nil for pointers instead of 0

--- a/sys/src/9/port/sysproc.c

+++ b/sys/src/9/port/sysproc.c

@@ -278,19 +278,18 @@

 			kstrdup(&elem, up->genbuf);

 		n = devtab[tc->type]->read(tc, &exec, sizeof(Exec), 0);

-		if(n < 2)

+		if(n <= 2)

 			error(Ebadexec);

-		magic = l2be(exec.magic);

-		text = l2be(exec.text);

-		entry = l2be(exec.entry);

-		if(n==sizeof(Exec) && (magic == AOUT_MAGIC)){

+		if(n == sizeof(Exec) && (magic = l2be(exec.magic)) == AOUT_MAGIC){

+			text = l2be(exec.text);

+			entry = l2be(exec.entry);

 			switch(magic){

 			case S_MAGIC:

 				text += 8;

-				align = 0x200000ull;	/* 2MB segment alignment for amd64 */

+				align = 0x200000;	/* 2MB segment alignment for amd64 */

 				break;

 			case V_MAGIC:

-				align = 0x4000ull;	/* MIPS has 16K page alignment */

+				align = 0x4000;		/* MIPS has 16K page alignment */

 				break;

 			}

 			if(text >= (USTKTOP-USTKSIZE)-(UTZERO+sizeof(Exec))

@@ -303,11 +302,11 @@

 		/*

 		 * Process #! /bin/sh args ...

 		 */

-		memmove(line, &exec, sizeof(Exec));

+		memmove(line, &exec, n);

 		if(indir || line[0]!='#' || line[1]!='!')

 			error(Ebadexec);

 		n = shargs(line, n, progarg);

-		if(n == 0)

+		if(n < 1)

 			error(Ebadexec);

 		indir = 1;

 		/*

@@ -314,7 +313,7 @@

 		 * First arg becomes complete file name

 		 */

 		progarg[n++] = file;

-		progarg[n] = 0;

+		progarg[n] = nil;

 		argp0++;

 		file = progarg[0];

 		if(strlen(elem) >= sizeof progelem)

@@ -539,11 +538,14 @@

 	s += 2;

 	n -= 2;		/* skip #! */

-	for(i=0; s[i]!='\n'; i++)

-		if(i == n-1)

+	for(i=0;; i++){

+		if(i >= n)

 			return 0;

+		if(s[i]=='\n')

+			break;

+	}

 	s[i] = 0;

-	*ap = 0;

+

 	i = 0;

 	for(;;) {

 		while(*s==' ' || *s=='\t')

@@ -550,16 +552,14 @@

 			s++;

 		if(*s == 0)

 			break;

-		i++;

-		*ap++ = s;

-		*ap = 0;

+		ap[i++] = s++;

 		while(*s && *s!=' ' && *s!='\t')

 			s++;

 		if(*s == 0)

 			break;

-		else

-			*s++ = 0;

+		*s++ = 0;

 	}

+	ap[i] = nil;

 	return i;

 }