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devmnt: do not use user buffer to update the mount cache

using the user buffer has a race where the user can modify
the buffer from another process before it is copied into the cache.
this allows poisoning the cache for every file where the user
has read access.

instead, we update the cache from kernel memory.

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

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

@@ -634,32 +634,11 @@

 mntread(Chan *c, void *buf, long n, vlong off)

 {

 	uchar *p, *e;

-	int nc, cache, isdir, dirlen;

+	int dirlen;

-	isdir = 0;

-	cache = c->flag & CCACHE;

-	if(c->qid.type & QTDIR) {

-		cache = 0;

-		isdir = 1;

-	}

-

 	p = buf;

-	if(cache) {

-		nc = cread(c, buf, n, off);

-		if(nc > 0) {

-			n -= nc;

-			if(n == 0)

-				return nc;

-			p += nc;

-			off += nc;

-		}

-		n = mntrdwr(Tread, c, p, n, off);

-		cupdate(c, p, n, off);

-		return n + nc;

-	}

-

-	n = mntrdwr(Tread, c, buf, n, off);

-	if(isdir) {

+	n = mntrdwr(Tread, c, p, n, off);

+	if(c->qid.type & QTDIR) {

 		for(e = &p[n]; p+BIT16SZ < e; p += dirlen){

 			dirlen = BIT16SZ+GBIT16(p);

 			if(p+dirlen > e)

@@ -695,6 +674,14 @@

 	if(c->qid.type & QTDIR)

 		cache = 0;

 	for(;;) {

+		if(cache && type == Tread) {

+			nr = cread(c, (uchar*)uba, n, off);

+			if(nr > 0) {

+				nreq = nr;

+				goto Next;

+			}

+		}

+

 		r = mntralloc(c, m->msize);

 		if(waserror()) {

 			mntfree(r);

@@ -714,13 +701,39 @@

 		if(nr > nreq)

 			nr = nreq;

+		if(cache) {

+			/*

+			 * note that we cannot update the cache from uba as

+			 * the user could change its contents from another

+			 * process before the data gets copied to the cached.

+			 */

+			if(type == Tread) {

+				ulong nc, nn;

+				Block *b;

+

+				nc = 0;

+				for(b = r->b; b != nil; b = b->next) {

+					nn = BLEN(b);

+					if(nc+nn > nr)

+						nn = nr - nc;

+					cupdate(c, b->rp, nn, off + nc);

+					nc += nn;

+					if(nc >= nr)

+						break;

+				}

+			} else {

+				if(convM2S(r->rpc, r->rpclen, &r->request) == 0)

+					panic("convM2S");

+				cwrite(c, (uchar*)r->request.data, nr, off);

+			}

+		}

+

 		if(type == Tread)

 			r->b = bl2mem((uchar*)uba, r->b, nr);

-		else if(cache)

-			cwrite(c, (uchar*)uba, nr, off);

-

-		poperror();

 		mntfree(r);

+		poperror();

+

+	Next:

 		off += nr;

 		uba += nr;

 		cnt += nr;

@@ -807,7 +820,6 @@

 			up->text, up->pid, n, r->request.tag, r->request.fid, r->request.type);

 		error(Emountrpc);

 	}

-		

 	if(devtab[m->c->type]->write(m->c, r->rpc, n, 0) != n)

 		error(Emountrpc);

-- 

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