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Fix a pointer arithmetic bug in memarea_alloc()

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Fortunately, the arithmetic cannot actually overflow, so long as we
*always* check for the size of potentially hostile input before
copying it.  I think we do, though.  We do check each line against
MAX_LINE_LENGTH, and each object name or object against
MAX_UNPARSED_OBJECT_SIZE, both of which are 128k.  So to get this
overflow, we need to have our memarea allocated way way too high up
in RAM, which most allocators won't actually do.

Bugfix on 0.2.1.1-alpha, where memarea was introduced.

Found by Guido Vranken.
This commit is contained in:
Nick Mathewson 2016-05-19 10:52:27 -04:00
parent 4165b1a0da
commit be2d37ad3c
2 changed files with 12 additions and 3 deletions
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7
changes/memarea_overflow Normal file
View File

@ -0,0 +1,7 @@
o Minor bugfixes (pointer arithmetic):
- Fix a bug in memarea_alloc() that could have resulted in remote heap
write access, if Tor had ever passed an unchecked size to
memarea_alloc(). Fortunately, all the sizes we pass to memarea_alloc()
are pre-checked to be less than 128 kilobytes. Fixes bug 19150; bugfix
on 0.2.1.1-alpha. Bug found by Guido Vranken.

8
src/common/memarea.c
View File

@ -80,8 +80,7 @@ typedef struct memarea_chunk_t {
struct memarea_chunk_t *next_chunk;
size_t mem_size; /**< How much RAM is available in mem, total? */
char *next_mem; /**< Next position in mem to allocate data at. If it's
* greater than or equal to mem+mem_size, this chunk is
* full. */
* equal to mem+mem_size, this chunk is full. */
#ifdef USE_ALIGNED_ATTRIBUTE
char mem[FLEXIBLE_ARRAY_MEMBER] __attribute__((aligned(MEMAREA_ALIGN)));
#else
@ -237,7 +236,10 @@ memarea_alloc(memarea_t *area, size_t sz)
tor_assert(sz < SIZE_T_CEILING);
if (sz == 0)
sz = 1;
if (chunk->next_mem+sz > chunk->U_MEM+chunk->mem_size) {
tor_assert(chunk->next_mem <= chunk->U_MEM + chunk->mem_size);
const size_t space_remaining =
(chunk->U_MEM + chunk->mem_size) - chunk->next_mem;
if (sz > space_remaining) {
if (sz+CHUNK_HEADER_SIZE >= CHUNK_SIZE) {
/* This allocation is too big. Stick it in a special chunk, and put
* that chunk second in the list. */