Re: [PATCH v8 05/12] test_sysfs: add support to use kernfs failure injection

From: Kees Cook
Date: Tue Oct 05 2021 - 15:51:39 EST

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On Mon, Sep 27, 2021 at 09:37:58AM -0700, Luis Chamberlain wrote:
> This extends test_sysfs with support for using the failure injection
> wait completion and knobs to force a few race conditions which
> demonstrates that kernfs active reference protection is sufficient
> for kobject / device protection at higher layers.
>
> This adds 4 new tests which tries to remove the device attribute
> store operation in 4 different situations:
>
> 1) at the start of kernfs_kernfs_fop_write_iter()
> 2) before the of->mutex is held in kernfs_kernfs_fop_write_iter()
> 3) after the of->mutex is held in kernfs_kernfs_fop_write_iter()
> 4) after the kernfs node active reference is taken
>
> A write fails in call cases except the last one, test number #32. There
> is a good explanation for this: *once* kernfs_get_active() gets called
> we have a guarantee that the kernfs entry cannot be removed. If
> kernfs_get_active() succeeds that entry cannot be removed and so
> anything trying to remove that entry will have to wait. It is perhaps
> not obvious but since a sysfs write will trigger eventually a
> kernfs_get_active() call, and *only* if this succeeds will the sysfs
> op be called, this and the fact that you cannot remove the kernfs
> entry while the kenfs entry is active implies that a module that
> created the respective sysfs / kernfs entry *cannot* possibly be
> removed during a sysfs operation. And test number 32 provides us with
> proof of this. If it were not true test #32 should crash.
>
> No null dereferences are reproduced, even though this has been observed
> in some complex testing cases [0]. If this issue really exists we should
> have enough tools on the sysfs_test toolbox now to try to reproduce
> this easily without having to poke around other drivers. It very likley
> was the case that the issue reported [0] was possibly a side issue after
> the first bug which was zram specific. This is why it is important to
> isolate the issue and try to reproduce it in a generic form using the
> test_sysfs driver.
>
> [0] https://lkml.kernel.org/r/20210623215007.862787-1-mcgrof@xxxxxxxxxx
>
> Signed-off-by: Luis Chamberlain <mcgrof@xxxxxxxxxx>
> ---
> lib/Kconfig.debug | 3 +
> lib/test_sysfs.c | 31 +++++
> tools/testing/selftests/sysfs/config | 3 +
> tools/testing/selftests/sysfs/sysfs.sh | 175 +++++++++++++++++++++++++
> 4 files changed, 212 insertions(+)
>
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index a29b7d398c4e..176b822654e5 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -2358,6 +2358,9 @@ config TEST_SYSFS
> depends on SYSFS
> depends on NET
> depends on BLOCK
> + select FAULT_INJECTION
> + select FAULT_INJECTION_DEBUG_FS
> + select FAIL_KERNFS_KNOBS

I don't like seeing "select" for user-configurable CONFIGs -- things
tend to end up weird. This should simply be:

depends on FAIL_KERNFS_KNOBS

> help
> This builds the "test_sysfs" module. This driver enables to test the
> sysfs file system safely without affecting production knobs which
> diff --git a/lib/test_sysfs.c b/lib/test_sysfs.c
> index 2043ca494af8..c6e62de61403 100644
> --- a/lib/test_sysfs.c
> +++ b/lib/test_sysfs.c
> @@ -38,6 +38,11 @@
> #include <linux/rtnetlink.h>
> #include <linux/genhd.h>
> #include <linux/blkdev.h>
> +#include <linux/kernfs.h>
> +
> +#ifdef CONFIG_FAIL_KERNFS_KNOBS

This isn't an optional config here (and following)?

> +MODULE_IMPORT_NS(KERNFS_DEBUG_PRIVATE);
> +#endif
>
> static bool enable_lock;
> module_param(enable_lock, bool_enable_only, 0644);
> @@ -82,6 +87,13 @@ static bool enable_verbose_rmmod;
> module_param(enable_verbose_rmmod, bool_enable_only, 0644);
> MODULE_PARM_DESC(enable_verbose_rmmod, "enable verbose print messages on rmmod");
>
> +#ifdef CONFIG_FAIL_KERNFS_KNOBS
> +static bool enable_completion_on_rmmod;
> +module_param(enable_completion_on_rmmod, bool_enable_only, 0644);
> +MODULE_PARM_DESC(enable_completion_on_rmmod,
> + "enable sending a kernfs completion on rmmod");
> +#endif
> +
> static int sysfs_test_major;
>
> /**
> @@ -285,6 +297,12 @@ static ssize_t config_show(struct device *dev,
> "enable_verbose_writes:\t%s\n",
> enable_verbose_writes ? "true" : "false");
>
> +#ifdef CONFIG_FAIL_KERNFS_KNOBS
> + len += snprintf(buf+len, PAGE_SIZE - len,
> + "enable_completion_on_rmmod:\t%s\n",
> + enable_completion_on_rmmod ? "true" : "false");
> +#endif
> +
> test_dev_config_unlock(test_dev);
>
> return len;
> @@ -904,10 +922,23 @@ static int __init test_sysfs_init(void)
> }
> module_init(test_sysfs_init);
>
> +#ifdef CONFIG_FAIL_KERNFS_KNOBS
> +/* The goal is to race our device removal with a pending kernfs -> store call */
> +static void test_sysfs_kernfs_send_completion_rmmod(void)
> +{
> + if (!enable_completion_on_rmmod)
> + return;
> + complete(&kernfs_debug_wait_completion);
> +}
> +#else
> +static inline void test_sysfs_kernfs_send_completion_rmmod(void) {}
> +#endif
> +
> static void __exit test_sysfs_exit(void)
> {
> if (enable_debugfs)
> debugfs_remove(debugfs_dir);
> + test_sysfs_kernfs_send_completion_rmmod();
> if (delay_rmmod_ms)
> msleep(delay_rmmod_ms);
> unregister_test_dev_sysfs(first_test_dev);
> diff --git a/tools/testing/selftests/sysfs/config b/tools/testing/selftests/sysfs/config
> index 9196f452ecd5..2876a229f95b 100644
> --- a/tools/testing/selftests/sysfs/config
> +++ b/tools/testing/selftests/sysfs/config
> @@ -1,2 +1,5 @@
> CONFIG_SYSFS=m
> CONFIG_TEST_SYSFS=m
> +CONFIG_FAULT_INJECTION=y
> +CONFIG_FAULT_INJECTION_DEBUG_FS=y
> +CONFIG_FAIL_KERNFS_KNOBS=y
> diff --git a/tools/testing/selftests/sysfs/sysfs.sh b/tools/testing/selftests/sysfs/sysfs.sh
> index b3f4c2236c7f..f928635d0e35 100755
> --- a/tools/testing/selftests/sysfs/sysfs.sh
> +++ b/tools/testing/selftests/sysfs/sysfs.sh
> @@ -62,6 +62,10 @@ ALL_TESTS="$ALL_TESTS 0025:1:1:test_dev_y:block"
> ALL_TESTS="$ALL_TESTS 0026:1:1:test_dev_y:block"
> ALL_TESTS="$ALL_TESTS 0027:1:0:test_dev_x:block" # deadlock test
> ALL_TESTS="$ALL_TESTS 0028:1:0:test_dev_x:block" # deadlock test with rntl_lock
> +ALL_TESTS="$ALL_TESTS 0029:1:1:test_dev_x:block" # kernfs race removal of store
> +ALL_TESTS="$ALL_TESTS 0030:1:1:test_dev_x:block" # kernfs race removal before mutex
> +ALL_TESTS="$ALL_TESTS 0031:1:1:test_dev_x:block" # kernfs race removal after mutex
> +ALL_TESTS="$ALL_TESTS 0032:1:1:test_dev_x:block" # kernfs race removal after active
>
> allow_user_defaults()
> {
> @@ -92,6 +96,9 @@ allow_user_defaults()
> if [ -z $SYSFS_DEBUGFS_DIR ]; then
> SYSFS_DEBUGFS_DIR="/sys/kernel/debug/test_sysfs"
> fi
> + if [ -z $KERNFS_DEBUGFS_DIR ]; then
> + KERNFS_DEBUGFS_DIR="/sys/kernel/debug/kernfs"
> + fi
> if [ -z $PAGE_SIZE ]; then
> PAGE_SIZE=$(getconf PAGESIZE)
> fi
> @@ -167,6 +174,14 @@ modprobe_reset_enable_rtnl_lock_on_rmmod()
> unset FIRST_MODPROBE_ARGS
> }
>
> +modprobe_reset_enable_completion()
> +{
> + FIRST_MODPROBE_ARGS="enable_completion_on_rmmod=1 enable_verbose_writes=1"
> + FIRST_MODPROBE_ARGS="$FIRST_MODPROBE_ARGS enable_verbose_rmmod=1 delay_rmmod_ms=0"
> + modprobe_reset
> + unset FIRST_MODPROBE_ARGS
> +}
> +
> load_req_mod()
> {
> modprobe_reset
> @@ -197,6 +212,63 @@ debugfs_reset_first_test_dev_ignore_errors()
> echo -n "1" >"$SYSFS_DEBUGFS_DIR"/reset_first_test_dev
> }
>
> +debugfs_kernfs_kernfs_fop_write_iter_exists()
> +{
> + KNOB_DIR="${KERNFS_DEBUGFS_DIR}/config_fail_kernfs_fop_write_iter"
> + if [[ ! -d $KNOB_DIR ]]; then
> + echo "kernfs debugfs does not exist $KNOB_DIR"
> + return 0;
> + fi
> + KNOB_DEBUGFS="${KERNFS_DEBUGFS_DIR}/fail_kernfs_fop_write_iter"
> + if [[ ! -d $KNOB_DEBUGFS ]]; then
> + echo -n "kernfs debugfs for coniguring fail_kernfs_fop_write_iter "
> + echo "does not exist $KNOB_DIR"
> + return 0;
> + fi
> + return 1
> +}
> +
> +debugfs_kernfs_kernfs_fop_write_iter_set_fail_once()
> +{
> + KNOB_DEBUGFS="${KERNFS_DEBUGFS_DIR}/fail_kernfs_fop_write_iter"
> + echo 1 > $KNOB_DEBUGFS/interval
> + echo 100 > $KNOB_DEBUGFS/probability
> + echo 0 > $KNOB_DEBUGFS/space
> + # Disable verbose messages on the kernel ring buffer which may
> + # confuse developers with a kernel panic.
> + echo 0 > $KNOB_DEBUGFS/verbose
> +
> + # Fail only once
> + echo 1 > $KNOB_DEBUGFS/times
> +}
> +
> +debugfs_kernfs_kernfs_fop_write_iter_set_fail_never()
> +{
> + KNOB_DEBUGFS="${KERNFS_DEBUGFS_DIR}/fail_kernfs_fop_write_iter"
> + echo 0 > $KNOB_DEBUGFS/times
> +}
> +
> +debugfs_kernfs_set_wait_ms()
> +{
> + SLEEP_AFTER_WAIT_MS="${KERNFS_DEBUGFS_DIR}/sleep_after_wait_ms"
> + echo $1 > $SLEEP_AFTER_WAIT_MS
> +}
> +
> +debugfs_kernfs_disable_wait_kernfs_fop_write_iter()
> +{
> + ENABLE_WAIT_KNOB="${KERNFS_DEBUGFS_DIR}/config_fail_kernfs_fop_write_iter/wait_"
> + for KNOB in ${ENABLE_WAIT_KNOB}*; do
> + echo 0 > $KNOB
> + done
> +}
> +
> +debugfs_kernfs_enable_wait_kernfs_fop_write_iter()
> +{
> + ENABLE_WAIT_KNOB="${KERNFS_DEBUGFS_DIR}/config_fail_kernfs_fop_write_iter/wait_$1"
> + echo -n "1" > $ENABLE_WAIT_KNOB
> + return $?
> +}
> +
> set_orig()
> {
> if [[ ! -z $TARGET ]] && [[ ! -z $ORIG ]]; then
> @@ -972,6 +1044,105 @@ sysfs_test_0028()
> fi
> }
>
> +sysfs_race_kernfs_kernfs_fop_write_iter()
> +{
> + TARGET="${DIR}/$(get_test_target $1)"
> + WAIT_AT=$2
> + EXPECT_WRITE_RETURNS=$3
> + MSDELAY=$4
> +
> + modprobe_reset_enable_completion
> + ORIG=$(cat "${TARGET}")
> + TEST_STR=$(( $ORIG + 1 ))
> +
> + echo -n "Test racing removal of sysfs store op with kernfs $WAIT_AT ... "
> +
> + if debugfs_kernfs_kernfs_fop_write_iter_exists; then
> + echo -n "skipping test as CONFIG_FAIL_KERNFS_KNOBS "
> + echo " or CONFIG_FAULT_INJECTION_DEBUG_FS is disabled"
> + return $ksft_skip
> + fi
> +
> + # Allow for failing the kernfs_kernfs_fop_write_iter call once,
> + # we'll provide exact context shortly afterwards.
> + debugfs_kernfs_kernfs_fop_write_iter_set_fail_once
> +
> + # First disable all waits
> + debugfs_kernfs_disable_wait_kernfs_fop_write_iter
> +
> + # Enable a wait_for_completion(&kernfs_debug_wait_completion) at the
> + # specified location inside the kernfs_fop_write_iter() routine
> + debugfs_kernfs_enable_wait_kernfs_fop_write_iter $WAIT_AT
> +
> + # Configure kernfs so that after its wait_for_completion() it
> + # will msleep() this amount of time and schedule(). We figure this
> + # will be sufficient time to allow for our module removal to complete.
> + debugfs_kernfs_set_wait_ms $MSDELAY
> +
> + # Now we trigger a kernfs write op, which will run kernfs_fop_write_iter,
> + # but will wait until our driver sends a respective completion
> + set_test_ignore_errors &
> + write_pid=$!
> +
> + # At this point kernfs_fop_write_iter() hasn't run our op, its
> + # waiting for our completion at the specified time $WAIT_AT.
> + # We now remove our module which will send a
> + # complete(&kernfs_debug_wait_completion) right before we deregister
> + # our device and the sysfs device attributes are removed.
> + #
> + # After the completion is sent, the test_sysfs driver races with
> + # kernfs to do the device deregistration with the kernfs msleep
> + # and schedule(). This should mean we've forced trying to remove the
> + # module prior to allowing kernfs to run our store operation. If the
> + # race did happen we'll panic with a null dereference on the store op.
> + #
> + # If no race happens we should see no write operation triggered.
> + modprobe -r $TEST_DRIVER > /dev/null 2>&1
> +
> + debugfs_kernfs_kernfs_fop_write_iter_set_fail_never
> +
> + wait $write_pid
> + if [[ $? -eq $EXPECT_WRITE_RETURNS ]]; then
> + echo "ok"
> + else
> + echo "FAIL" >&2
> + fi
> +}
> +
> +sysfs_test_0029()
> +{
> + for delay in 0 2 4 8 16 32 64 128 246 512 1024; do
> + echo "Using delay-after-completion: $delay"
> + sysfs_race_kernfs_kernfs_fop_write_iter 0029 at_start 1 $delay
> + done
> +}
> +
> +sysfs_test_0030()
> +{
> + for delay in 0 2 4 8 16 32 64 128 246 512 1024; do
> + echo "Using delay-after-completion: $delay"
> + sysfs_race_kernfs_kernfs_fop_write_iter 0030 before_mutex 1 $delay
> + done
> +}
> +
> +sysfs_test_0031()
> +{
> + for delay in 0 2 4 8 16 32 64 128 246 512 1024; do
> + echo "Using delay-after-completion: $delay"
> + sysfs_race_kernfs_kernfs_fop_write_iter 0031 after_mutex 1 $delay
> + done
> +}
> +
> +# A write only succeeds *iff* a module removal happens *after* the
> +# kernfs active reference is obtained with kernfs_get_active().
> +sysfs_test_0032()
> +{
> + for delay in 0 2 4 8 16 32 64 128 246 512 1024; do
> + echo "Using delay-after-completion: $delay"
> + sysfs_race_kernfs_kernfs_fop_write_iter 0032 after_active 0 $delay
> + done
> +}
> +
> test_gen_desc()
> {
> echo -n "$1 x $(get_test_count $1)"
> @@ -1013,6 +1184,10 @@ list_tests()
> echo "$(test_gen_desc 0026) - block test writing y larger delay and resetting device"
> echo "$(test_gen_desc 0027) - test rmmod deadlock while writing x ... "
> echo "$(test_gen_desc 0028) - test rmmod deadlock using rtnl_lock while writing x ..."
> + echo "$(test_gen_desc 0029) - racing removal of store op with kernfs at start"
> + echo "$(test_gen_desc 0030) - racing removal of store op with kernfs before mutex"
> + echo "$(test_gen_desc 0031) - racing removal of store op with kernfs after mutex"
> + echo "$(test_gen_desc 0032) - racing removal of store op with kernfs after active"
> }
>
> usage()
> --
> 2.30.2
>

--
Kees Cook

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