[PATCH 00/12] slab: Introduce kmalloc_size_roundup()

Kees Cook
2022-09-22T03:10:23Z

Hi,

This series fixes up the cases where callers of ksize() use it to
opportunistically grow their buffer sizes, which can run afoul of the
__alloc_size hinting that CONFIG_UBSAN_BOUNDS and CONFIG_FORTIFY_SOURCE
use to perform dynamic buffer bounds checking. Quoting the first patch:

In the effort to help the compiler reason about buffer sizes, the
__alloc_size attribute was added to allocators. This improves the scope
of the compiler's ability to apply CONFIG_UBSAN_BOUNDS and (in the near
future) CONFIG_FORTIFY_SOURCE. For most allocations, this works well,
as the vast majority of callers are not expecting to use more memory
than what they asked for.

There is, however, one common exception to this: anticipatory resizing
of kmalloc allocations. These cases all use ksize() to determine the
actual bucket size of a given allocation (e.g. 128 when 126 was asked
for). This comes in two styles in the kernel:

1) An allocation has been determined to be too small, and needs to be
resized. Instead of the caller choosing its own next best size, it
wants to minimize the number of calls to krealloc(), so it just uses
ksize() plus some additional bytes, forcing the realloc into the next
bucket size, from which it can learn how large it is now. For example:

data = krealloc(data, ksize(data) + 1, gfp);
data_len = ksize(data);

2) The minimum size of an allocation is calculated, but since it may
grow in the future, just use all the space available in the chosen
bucket immediately, to avoid needing to reallocate later. A good
example of this is skbuff's allocators:

data = kmalloc_reserve(size, gfp_mask, node, &pfmemalloc);
...
/* kmalloc(size) might give us more room than requested.
* Put skb_shared_info exactly at the end of allocated zone,
* to allow max possible filling before reallocation.
*/
osize = ksize(data);
size = SKB_WITH_OVERHEAD(osize);

In both cases, the "how large is the allocation?" question is answered
_after_ the allocation, where the compiler hinting is not in an easy place
to make the association any more. This mismatch between the compiler's
view of the buffer length and the code's intention about how much it is
going to actually use has already caused problems[1]. It is possible to
fix this by reordering the use of the "actual size" information.

We can serve the needs of users of ksize() and still have accurate buffer
length hinting for the compiler by doing the bucket size calculation
_before_ the allocation. Code can instead ask "how large an allocation
would I get for a given size?".

Introduce kmalloc_size_roundup(), to serve this function so we can start
replacing the "anticipatory resizing" uses of ksize().

[1] https://github.com/ClangBuiltLinux/linux/issues/1599
https://github.com/KSPP/linux/issues/183
-------

And after adding kmalloc_size_roundup(), put it to use with the various
ksize() callers, restore the previously removed __alloc_size hint,
and fix the use of __malloc annotations.

I tried to trim the CC list on this series since it got rather long. I
kept all the suggested mailing lists, though. :)

Thanks!

-Kees

Kees Cook (12):
slab: Introduce kmalloc_size_roundup()
skbuff: Proactively round up to kmalloc bucket size
net: ipa: Proactively round up to kmalloc bucket size
btrfs: send: Proactively round up to kmalloc bucket size
dma-buf: Proactively round up to kmalloc bucket size
coredump: Proactively round up to kmalloc bucket size
igb: Proactively round up to kmalloc bucket size
openvswitch: Proactively round up to kmalloc bucket size
x86/microcode/AMD: Track patch allocation size explicitly
iwlwifi: Track scan_cmd allocation size explicitly
slab: Remove __malloc attribute from realloc functions
slab: Restore __alloc_size attribute to __kmalloc_track_caller

arch/x86/include/asm/microcode.h | 1 +
arch/x86/kernel/cpu/microcode/amd.c | 3 +-
drivers/dma-buf/dma-resv.c | 9 +++-
drivers/net/ethernet/intel/igb/igb_main.c | 1 +
drivers/net/ipa/gsi_trans.c | 7 ++-
drivers/net/wireless/intel/iwlwifi/dvm/dev.h | 1 +
drivers/net/wireless/intel/iwlwifi/dvm/scan.c | 10 +++-
drivers/net/wireless/intel/iwlwifi/mvm/mvm.h | 3 +-
drivers/net/wireless/intel/iwlwifi/mvm/ops.c | 3 +-
drivers/net/wireless/intel/iwlwifi/mvm/scan.c | 6 +--
fs/btrfs/send.c | 11 +++--
fs/coredump.c | 7 ++-
include/linux/compiler_types.h | 13 ++----
include/linux/slab.h | 46 ++++++++++++++++---
mm/slab_common.c | 17 +++++++
net/core/skbuff.c | 34 +++++++-------
net/openvswitch/flow_netlink.c | 4 +-
17 files changed, 125 insertions(+), 51 deletions(-)

--
2.34.1