Subject: CVE-2023-52452: bpf: Fix accesses to uninit stack slots

From: Greg Kroah-Hartman <[email-masked]>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

bpf: Fix accesses to uninit stack slots

Privileged programs are supposed to be able to read uninitialized stack
memory (ever since 6715df8d5) but, before this patch, these accesses
were permitted inconsistently. In particular, accesses were permitted
above state->allocated_stack, but not below it. In other words, if the
stack was already "large enough", the access was permitted, but
otherwise the access was rejected instead of being allowed to "grow the
stack". This undesired rejection was happening in two places:
- in check_stack_slot_within_bounds()
- in check_stack_range_initialized()
This patch arranges for these accesses to be permitted. A bunch of tests
that were relying on the old rejection had to change; all of them were
changed to add also run unprivileged, in which case the old behavior
persists. One tests couldn't be updated - global_func16 - because it
can't run unprivileged for other reasons.

This patch also fixes the tracking of the stack size for variable-offset
reads. This second fix is bundled in the same commit as the first one
because they're inter-related. Before this patch, writes to the stack
using registers containing a variable offset (as opposed to registers
with fixed, known values) were not properly contributing to the
function's needed stack size. As a result, it was possible for a program
to verify, but then to attempt to read out-of-bounds data at runtime
because a too small stack had been allocated for it.

Each function tracks the size of the stack it needs in
bpf_subprog_info.stack_depth, which is maintained by
update_stack_depth(). For regular memory accesses, check_mem_access()
was calling update_state_depth() but it was passing in only the fixed
part of the offset register, ignoring the variable offset. This was
incorrect; the minimum possible value of that register should be used
instead.

This tracking is now fixed by centralizing the tracking of stack size in
grow_stack_state(), and by lifting the calls to grow_stack_state() to
check_stack_access_within_bounds() as suggested by Andrii. The code is
now simpler and more convincingly tracks the correct maximum stack size.
check_stack_range_initialized() can now rely on enough stack having been
allocated for the access; this helps with the fix for the first issue.

A few tests were changed to also check the stack depth computation. The
one that fails without this patch is verifier_var_off:stack_write_priv_vs_unpriv.

The Linux kernel CVE team has assigned CVE-2023-52452 to this issue.

Affected and fixed versions
===========================

	Issue introduced in 5.12 with commit 01f810ace9ed and fixed in 6.6.14 with commit 0954982db828
	Issue introduced in 5.12 with commit 01f810ace9ed and fixed in 6.7.2 with commit fbcf372c8eda
	Issue introduced in 5.12 with commit 01f810ace9ed and fixed in 6.8 with commit 6b4a64bafd10
	Issue introduced in 5.10.33 with commit f3c4b01689d3
	Issue introduced in 5.11.17 with commit d1b725ea5d10

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2023-52452
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	kernel/bpf/verifier.c
	tools/testing/selftests/bpf/progs/iters.c
	tools/testing/selftests/bpf/progs/test_global_func16.c
	tools/testing/selftests/bpf/progs/verifier_basic_stack.c
	tools/testing/selftests/bpf/progs/verifier_int_ptr.c
	tools/testing/selftests/bpf/progs/verifier_raw_stack.c
	tools/testing/selftests/bpf/progs/verifier_var_off.c
	tools/testing/selftests/bpf/verifier/atomic_cmpxchg.c
	tools/testing/selftests/bpf/verifier/calls.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/0954982db8283016bf38e9db2da5adf47a102e19
	https://git.kernel.org/stable/c/fbcf372c8eda2290470268e0afb5ab5d5f5d5fde
	https://git.kernel.org/stable/c/6b4a64bafd107e521c01eec3453ce94a3fb38529