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buflib: add paranoia checks for handles

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Handle checks ensure that the data in the handle table points
within buflib memory and checks handle entry pointers in block
headers before dereferencing them.

Change-Id: Ic16f1b81c1a0ea63c0e7f48d87938293b75c2419
This commit is contained in:
Aidan MacDonald 2022-03-30 17:39:32 +01:00
parent 73b9b227eb
commit f82f3044a7
1 changed files with 105 additions and 18 deletions
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123
firmware/buflib.c
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@ -98,7 +98,9 @@
#define BPANICF panicf
/* Available paranoia checks */
#define PARANOIA_CHECK_LENGTH (1 << 0)
#define PARANOIA_CHECK_LENGTH (1 << 0)
#define PARANOIA_CHECK_HANDLE (1 << 1)
#define PARANOIA_CHECK_BLOCK_HANDLE (1 << 2)
/* Bitmask of enabled paranoia checks */
#define BUFLIB_PARANOIA 0
@ -143,6 +145,26 @@ static union buflib_data* find_block_before(struct buflib_context *ctx,
static void check_block_length(struct buflib_context *ctx,
union buflib_data *block);
/* Check a handle table entry to ensure the user pointer is within the
* bounds of the allocated area and there is enough room for a minimum
* size block header.
*
* This verifies that it is safe to convert the entry's pointer to a
* block end pointer and dereference fields at the block end.
*/
static void check_handle(struct buflib_context *ctx,
union buflib_data *h_entry);
/* Check a block's handle pointer to ensure it is within the handle
* table, and that the user pointer is pointing within the block.
*
* This verifies that it is safe to dereference the entry, in addition
* to all checks performed by check_handle(). It also ensures that the
* pointer in the handle table points within the block, as determined
* by the length field at the start of the block.
*/
static void check_block_handle(struct buflib_context *ctx,
union buflib_data *block);
/* Initialize buffer manager */
void
@ -271,15 +293,28 @@ void handle_free(struct buflib_context *ctx, union buflib_data *handle)
static inline
union buflib_data* handle_to_block(struct buflib_context* ctx, int handle)
{
union buflib_data *data = ALIGN_DOWN(buflib_get_data(ctx, handle), sizeof (*data));
/* this is a valid case, e.g. during buflib_alloc_ex() when the handle
* has already been allocated but not the data */
if (!data)
void *ptr = buflib_get_data(ctx, handle);
/* this is a valid case for shrinking if handle
* was freed by the shrink callback */
if (!ptr)
return NULL;
union buflib_data *data = ALIGN_DOWN(ptr, sizeof(*data));
return data - data[-bidx_BSIZE].val;
}
/* Get the block end pointer from a handle table entry */
static union buflib_data*
h_entry_to_block_end(struct buflib_context *ctx, union buflib_data *h_entry)
{
check_handle(ctx, h_entry);
void *alloc = h_entry->alloc;
union buflib_data *data = ALIGN_DOWN(alloc, sizeof(*data));
return data;
}
/* Shrink the handle table, returning true if its size was reduced, false if
* not
*/
@ -299,13 +334,6 @@ static inline bool handle_table_shrink(struct buflib_context *ctx)
return handle != old_last;
}
static inline
union buflib_data* userpointer_to_block_end(void *userpointer)
{
union buflib_data *data = ALIGN_DOWN(userpointer, sizeof(*data));
return data;
}
static uint32_t calc_block_crc(union buflib_data *block,
union buflib_data *block_end)
{
@ -327,11 +355,9 @@ move_block(struct buflib_context* ctx, union buflib_data* block, int shift)
char* new_start;
union buflib_data *new_block;
if (block < ctx->buf_start || block > ctx->alloc_end)
buflib_panic(ctx, "buflib data corrupted %p", block);
check_block_handle(ctx, block);
union buflib_data *h_entry = block[fidx_HANDLE].handle;
union buflib_data *block_end = userpointer_to_block_end(h_entry->alloc);
union buflib_data *block_end = h_entry_to_block_end(ctx, h_entry);
uint32_t crc = calc_block_crc(block, block_end);
if (crc != block_end[-bidx_CRC].crc)
@ -481,6 +507,7 @@ buflib_compact_and_shrink(struct buflib_context *ctx, unsigned shrink_hints)
if (!ops || !ops->shrink_callback)
continue;
check_block_handle(ctx, this);
union buflib_data* h_entry = this[fidx_HANDLE].handle;
int handle = ctx->handle_table - h_entry;
@ -949,6 +976,7 @@ buflib_shrink(struct buflib_context* ctx, int handle, void* new_start, size_t ne
new_block = aligned_newstart - metadata_size.val;
block[fidx_LEN].val = new_next_block - new_block;
check_block_handle(ctx, block);
block[fidx_HANDLE].handle->alloc = newstart;
if (block != new_block)
{
@ -971,7 +999,7 @@ buflib_shrink(struct buflib_context* ctx, int handle, void* new_start, size_t ne
/* update crc of the metadata */
union buflib_data *new_h_entry = new_block[fidx_HANDLE].handle;
union buflib_data *new_block_end = userpointer_to_block_end(new_h_entry->alloc);
union buflib_data *new_block_end = h_entry_to_block_end(ctx, new_h_entry);
new_block_end[-bidx_CRC].crc = calc_block_crc(new_block, new_block_end);
/* Now deal with size changes that create free blocks after the allocation */
@ -1024,8 +1052,9 @@ void buflib_check_valid(struct buflib_context *ctx)
if (block->val < 0)
continue;
check_block_handle(ctx, block);
union buflib_data *h_entry = block[fidx_HANDLE].handle;
union buflib_data *block_end = userpointer_to_block_end(h_entry->alloc);
union buflib_data *block_end = h_entry_to_block_end(ctx, h_entry);
uint32_t crc = calc_block_crc(block, block_end);
if (crc != block_end[-bidx_CRC].crc)
{
@ -1130,3 +1159,61 @@ static void check_block_length(struct buflib_context *ctx,
}
}
}
static void check_handle(struct buflib_context *ctx,
union buflib_data *h_entry)
{
if (BUFLIB_PARANOIA & PARANOIA_CHECK_HANDLE)
{
/* For the pointer to be valid there needs to be room for a minimum
* size block header, so we add BUFLIB_NUM_FIELDS to ctx->buf_start. */
void *alloc = h_entry->alloc;
void *alloc_begin = ctx->buf_start + BUFLIB_NUM_FIELDS;
void *alloc_end = ctx->alloc_end;
/* buflib allows zero length allocations, so alloc_end is inclusive */
if (alloc < alloc_begin || alloc > alloc_end)
{
buflib_panic(ctx, "alloc outside buf [%p]=%p, %p-%p",
h_entry, alloc, alloc_begin, alloc_end);
}
}
}
static void check_block_handle(struct buflib_context *ctx,
union buflib_data *block)
{
if (BUFLIB_PARANOIA & PARANOIA_CHECK_BLOCK_HANDLE)
{
intptr_t length = block[fidx_LEN].val;
union buflib_data *h_entry = block[fidx_HANDLE].handle;
/* Check the handle pointer is properly aligned */
/* TODO: Can we ensure the compiler doesn't optimize this out?
* I dunno, maybe the compiler can assume the pointer is always
* properly aligned due to some C standard voodoo?? */
if (!IS_ALIGNED((uintptr_t)h_entry, alignof(*h_entry)))
{
buflib_panic(ctx, "handle unaligned [%p]=%p",
&block[fidx_HANDLE], h_entry);
}
/* Check the pointer is actually inside the handle table */
if (h_entry < ctx->last_handle || h_entry >= ctx->handle_table)
{
buflib_panic(ctx, "handle out of bounds [%p]=%p",
&block[fidx_HANDLE], h_entry);
}
/* Now check the allocation is within the block.
* This is stricter than check_handle(). */
void *alloc = h_entry->alloc;
void *alloc_begin = block;
void *alloc_end = block + length;
/* buflib allows zero length allocations, so alloc_end is inclusive */
if (alloc < alloc_begin || alloc > alloc_end)
{
buflib_panic(ctx, "alloc outside block [%p]=%p, %p-%p",
h_entry, alloc, alloc_begin, alloc_end);
}
}
}