Re: [Libguestfs] [libnbd PATCH] common/utils/vector: Better overflow handling
On 11/08/21 20:56, Eric Blake wrote:
Check newcap * itemsize for overflow prior to calling realloc, so
that
we don't accidentally truncate an existing array. Set errno to ENOMEM
on all failure paths, rather than leaving it indeterminate on
overflow. The patch works by assuming a prerequisite that
v->cap*itemsize is always smaller than size_t.
Fixes: 985dfa72ae (common/utils/vector.c: Optimize vector append)
---
Unless you see problems in this, I'll push this and also port it to
nbdkit.
common/utils/vector.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)
diff --git a/common/utils/vector.c b/common/utils/vector.c
index a4b43ce..c37f0c3 100644
--- a/common/utils/vector.c
+++ b/common/utils/vector.c
@@ -1,5 +1,5 @@
/* nbdkit
- * Copyright (C) 2018-2020 Red Hat Inc.
+ * Copyright (C) 2018-2021 Red Hat Inc.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are
@@ -44,12 +44,14 @@ generic_vector_reserve (struct generic_vector *v, size_t n, size_t
itemsize)
size_t reqcap, newcap;
reqcap = v->cap + n;
- if (reqcap < v->cap)
+ if (reqcap * itemsize < v->cap * itemsize) {
In my opinion, this is not good enough. Example (assuming uint64_t for
size_t):
itemsize = 2;
v->cap = 1;
n = 0x8000_0000_0000_0001;
therefore
reqcap = 0x8000_0000_0000_0002;
The product on the LHS will overflow, to 4. The product on the RHS will
be 2. So the controlling expression will evaluate to false.
+ errno = ENOMEM;
return -1; /* overflow */
+ }
newcap = (v->cap * 3 + 1) / 2;
This is another unchecked multiplication. What guarantees (v->cap * 3)
cannot overflow?
I strongly dislike attempts to catch overflows after the fact. I think
it's much better to ensure in advance that the addition or
multiplication cannot overflow. That usually involces subtraction,
division, and comparison. I assume generic_vector_reserve() is not on
any "hot path".
size_t reqcap, maxcap, newcap;
if (itemsize == 0) {
errno = EINVAL;
return -1;
}
if (n > (size_t)-1 - v->cap) {
errno = ENOMEM;
return -1; /* overflow */
}
reqcap = v->cap + n; /* can never overflow */
maxcap = (size_t)-1 / itemsize; /* never divides by zero */
if (reqcap > maxcap) {
errno = ENOMEM;
return -1; /* overflow */
}
if (v->cap > (size_t)-1 / 3 ||
v->cap * 3 > (size_t)-1 - 1) {
/* we cannot compute the next auto-increment; stick with the
* requested capacity
*/
newcap = reqcap;
} else {
newcap = v->cap * 3 + 1; /* cannot overflow */
newcap /= 2;
/* if the auto-increment is smaller than requested, or the
* auto-increment is too large as a number of plain bytes, then
* stick with the requested capacity */
if (newcap < reqcap || newcap > maxcap)
newcap = reqcap;
}
assert (newcap <= maxcap);
newptr = realloc (v->ptr, newcap * itemsize); /* cannot overflow */
/* ... */
Thanks
Laszlo
- if (newcap < reqcap)
+ if (newcap * itemsize < reqcap * itemsize)
newcap = reqcap;
newptr = realloc (v->ptr, newcap * itemsize);