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Johannes Berg d904d7064f um: time-travel: fix signal blocking race/hang 
[ Upstream commit 2cf3a3c4b8 ]

When signals are hard-blocked in order to do time-travel
socket processing, we set signals_blocked and then handle
SIGIO signals by setting the SIGIO bit in signals_pending.
When unblocking, we first set signals_blocked to 0, and
then handle all pending signals. We have to set it first,
so that we can again properly block/unblock inside the
unblock, if the time-travel handlers need to be processed.

Unfortunately, this is racy. We can get into this situation:

// signals_pending = SIGIO_MASK

unblock_signals_hard()
   signals_blocked = 0;
   if (signals_pending && signals_enabled) {
     block_signals();
     unblock_signals()
       ...
       sig_handler_common(SIGIO, NULL, NULL);
         sigio_handler()
           ...
           sigio_reg_handler()
             irq_do_timetravel_handler()
               reg->timetravel_handler() ==
               vu_req_interrupt_comm_handler()
                 vu_req_read_message()
                   vhost_user_recv_req()
                     vhost_user_recv()
                       vhost_user_recv_header()
                         // reads 12 bytes header of
                         // 20 bytes message
<-- receive SIGIO here <--
sig_handler()
   int enabled = signals_enabled; // 1
   if ((signals_blocked || !enabled) && (sig == SIGIO)) {
     if (!signals_blocked && time_travel_mode == TT_MODE_EXTERNAL)
       sigio_run_timetravel_handlers()
         _sigio_handler()
           sigio_reg_handler()
             ... as above ...
               vhost_user_recv_header()
                 // reads 8 bytes that were message payload
                 // as if it were header - but aborts since
                 // it then gets -EAGAIN
...
--> end signal handler -->
                       // continue in vhost_user_recv()
                       // full_read() for 8 bytes payload busy loops
                       // entire process hangs here

Conceptually, to fix this, we need to ensure that the
signal handler cannot run while we hard-unblock signals.
The thing that makes this more complex is that we can be
doing hard-block/unblock while unblocking. Introduce a
new signals_blocked_pending variable that we can keep at
non-zero as long as pending signals are being processed,
then we only need to ensure it's decremented safely and
the signal handler will only increment it if it's already
non-zero (or signals_blocked is set, of course.)

Note also that only the outermost call to hard-unblock is
allowed to decrement signals_blocked_pending, since it
could otherwise reach zero in an inner call, and leave
the same race happening if the timetravel_handler loops,
but that's basically required of it.

Fixes: d6b399a0e0 ("um: time-travel/signals: fix ndelay() in interrupt")
Link: https://patch.msgid.link/20240703110144.28034-2-johannes@sipsolutions.net
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
History 2024-08-03 08:49:47 +02:00
arch um: time-travel: fix signal blocking race/hang 2024-08-03 08:49:47 +02:00
block block: initialize integrity buffer to zero before writing it to media 2024-08-03 08:48:53 +02:00
certs certs: Fix build error when PKCS#11 URI contains semicolon 2023-02-09 11:28:11 +01:00
crypto crypto: aead,cipher - zeroize key buffer after use 2024-07-11 12:47:05 +02:00
Documentation dt-bindings: thermal: correct thermal zone node name limit 2024-08-03 08:49:30 +02:00
drivers phy: cadence-torrent: Check return value on register read 2024-08-03 08:49:47 +02:00
fs jfs: Fix array-index-out-of-bounds in diFree 2024-08-03 08:49:47 +02:00
include PCI: Introduce cleanup helpers for device reference counts and locks 2024-08-03 08:49:46 +02:00
init smp: Provide 'setup_max_cpus' definition on UP too 2024-06-16 13:41:42 +02:00
io_uring io_uring/io-wq: limit retrying worker initialisation 2024-08-03 08:49:46 +02:00
ipc ipc: fix memory leak in init_mqueue_fs() 2022-12-31 13:32:01 +01:00
kernel kdb: Use the passed prompt in kdb_position_cursor() 2024-08-03 08:49:47 +02:00
lib kobject_uevent: Fix OOB access within zap_modalias_env() 2024-08-03 08:49:39 +02:00
LICENSES LICENSES/LGPL-2.1: Add LGPL-2.1-or-later as valid identifiers 2021-12-16 14:33:10 +01:00
mm mm/numa_balancing: teach mpol_to_str about the balancing mode 2024-08-03 08:49:40 +02:00
net wifi: mac80211: check basic rates validity 2024-08-03 08:49:47 +02:00
rust rust: macros: fix soundness issue in module! macro 2024-05-17 11:56:17 +02:00
samples work around gcc bugs with 'asm goto' with outputs 2024-02-23 09:12:28 +01:00
scripts kbuild: Fix '-S -c' in x86 stack protector scripts 2024-08-03 08:49:39 +02:00
security task_work: s/task_work_cancel()/task_work_cancel_func()/ 2024-08-03 08:49:33 +02:00
sound ASoC: amd: yc: Support mic on Lenovo Thinkpad E16 Gen 2 2024-08-03 08:49:39 +02:00
tools selftests/sigaltstack: Fix ppc64 GCC build 2024-08-03 08:49:43 +02:00
usr usr/gen_init_cpio.c: remove unnecessary -1 values from int file 2022-10-03 14:21:44 -07:00
virt KVM: Fix a data race on last_boosted_vcpu in kvm_vcpu_on_spin() 2024-06-27 13:46:21 +02:00
.clang-format inet: ping: use hlist_nulls rcu iterator during lookup 2022-12-01 12:42:46 +01:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.get_maintainer.ignore get_maintainer: add Alan to .get_maintainer.ignore 2022-08-20 15:17:44 -07:00
.gitattributes .gitattributes: use 'dts' diff driver for dts files 2019-12-04 19:44:11 -08:00
.gitignore Kbuild: add Rust support 2022-09-28 09:02:20 +02:00
.mailmap 9 hotfixes. 6 for MM, 3 for other areas. Four of these patches address 2022-12-10 17:10:52 -08:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: Remove Michal Marek from Kbuild maintainers 2022-11-16 14:53:00 +09:00
Kbuild Kbuild updates for v6.1 2022-10-10 12:00:45 -07:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS MAINTAINERS: add leah to 6.1 MAINTAINERS file 2024-05-17 11:56:16 +02:00
Makefile Linux 6.1.102 2024-07-27 11:32:20 +02:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

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.