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Jann Horn 427cde23f7 exit: Put an upper limit on how often we can oops 
commit d4ccd54d28 upstream.

Many Linux systems are configured to not panic on oops; but allowing an
attacker to oops the system **really** often can make even bugs that look
completely unexploitable exploitable (like NULL dereferences and such) if
each crash elevates a refcount by one or a lock is taken in read mode, and
this causes a counter to eventually overflow.

The most interesting counters for this are 32 bits wide (like open-coded
refcounts that don't use refcount_t). (The ldsem reader count on 32-bit
platforms is just 16 bits, but probably nobody cares about 32-bit platforms
that much nowadays.)

So let's panic the system if the kernel is constantly oopsing.

The speed of oopsing 2^32 times probably depends on several factors, like
how long the stack trace is and which unwinder you're using; an empirically
important one is whether your console is showing a graphical environment or
a text console that oopses will be printed to.
In a quick single-threaded benchmark, it looks like oopsing in a vfork()
child with a very short stack trace only takes ~510 microseconds per run
when a graphical console is active; but switching to a text console that
oopses are printed to slows it down around 87x, to ~45 milliseconds per
run.
(Adding more threads makes this faster, but the actual oops printing
happens under &die_lock on x86, so you can maybe speed this up by a factor
of around 2 and then any further improvement gets eaten up by lock
contention.)

It looks like it would take around 8-12 days to overflow a 32-bit counter
with repeated oopsing on a multi-core X86 system running a graphical
environment; both me (in an X86 VM) and Seth (with a distro kernel on
normal hardware in a standard configuration) got numbers in that ballpark.

12 days aren't *that* short on a desktop system, and you'd likely need much
longer on a typical server system (assuming that people don't run graphical
desktop environments on their servers), and this is a *very* noisy and
violent approach to exploiting the kernel; and it also seems to take orders
of magnitude longer on some machines, probably because stuff like EFI
pstore will slow it down a ton if that's active.

Signed-off-by: Jann Horn <jannh@google.com>
Link: https://lore.kernel.org/r/20221107201317.324457-1-jannh@google.com
Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20221117234328.594699-2-keescook@chromium.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
2023-04-19 17:59:00 +08:00
arch ia64: make IA64_MCA_RECOVERY bool instead of tristate 2023-04-19 17:59:00 +08:00
block block: mq-deadline: Rename deadline_is_seq_writes() 2023-04-19 17:58:48 +08:00
certs
crypto
Documentation exit: Put an upper limit on how often we can oops 2023-04-19 17:59:00 +08:00
drivers drm/i915: Remove unused variable 2023-04-19 17:58:59 +08:00
fs sysctl: add a new register_sysctl_init() interface 2023-04-19 17:58:59 +08:00
include exit: Add and use make_task_dead. 2023-04-19 17:59:00 +08:00
init
io_uring io_uring/rw: remove leftover debug statement 2023-04-19 17:58:49 +08:00
ipc
kernel exit: Put an upper limit on how often we can oops 2023-04-19 17:59:00 +08:00
lib ubsan: no need to unset panic_on_warn in ubsan_epilogue() 2023-04-19 17:58:59 +08:00
LICENSES
mm kasan: no need to unset panic_on_warn in end_report() 2023-04-19 17:58:59 +08:00
net tcp: fix rate_app_limited to default to 1 2023-04-19 17:58:57 +08:00
samples
scripts
security tomoyo: fix broken dependency on *.conf.default 2023-04-19 17:58:50 +08:00
sound ASoC: fsl-asoc-card: Fix naming of AC'97 CODEC widgets 2023-04-19 17:58:58 +08:00
tools objtool: Add a missing comma to avoid string concatenation 2023-04-19 17:59:00 +08:00
usr
virt
.clang-format
.cocciconfig
.get_maintainer.ignore
.gitattributes
.gitignore
.mailmap
COPYING
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Kconfig
MAINTAINERS
Makefile Linux 5.15.90 2023-04-19 17:58:49 +08:00
README

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.