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lib: graph: fix vector_remove()

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vector_remove would corrupt the data in the following sequence:
1. assume vector v = [a, b], active = 2
2. vector_unset(v, 0) => v = [NULL, b], active = 2
3. vector_remove(v, 1)

vector_remove calls vector_unset(v, 1), vector_unset notices index #0 is
also NULL and thus sets active to 0.

The equality test in vector_remove() now fails, leading it to decrement
v->active *again*, leading to an underflow that will likely crash the
daemon (and might even be exploitable).

This call sequence does not happen in existing code since vector_unset()
is not used on graph from/to lists.  Nonetheless this is a buried land
mine in the code at best.

Rewrite the function - while we're at it, there's no reason to move the
entire array around, just fill the hole with the last element.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
Cc: Quentin Young <qlyoung@cumulusnetworks.com>
This commit is contained in:
David Lamparter 2017-01-26 05:59:50 +01:00
parent 5bf313994d
commit c30420200b
1 changed files with 9 additions and 4 deletions
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13
lib/graph.c
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@ -55,11 +55,16 @@ graph_new_node (struct graph *graph, void *data, void (*del) (void*))
static void static void
vector_remove (vector v, unsigned int ix) vector_remove (vector v, unsigned int ix)
{ {
vector_unset (v, ix); if (ix >= v->active)
if (ix == vector_active (v)) return; return;
for (; ix < vector_active (v) - 1; ix++)
v->index[ix] = v->index[ix+1]; /* v->active is guaranteed >= 1 because ix can't be lower than 0
* and v->active is > ix. */
v->active--; v->active--;
/* if ix == v->active--, we set the item to itself, then to NULL...
* still correct, no check neccessary. */
v->index[ix] = v->index[v->active];
v->index[v->active] = NULL;
} }
void void