On Wed, May 13, 2020 at 11:00:26AM -0400, Rafael Aquini wrote:
> Analogously to the introduction of panic_on_warn, this patch
> introduces a kernel option named panic_on_taint in order to
> provide a simple and generic way to stop execution and catch
> a coredump when the kernel gets tainted by any given taint flag.
>
> This is useful for debugging sessions as it avoids rebuilding
> the kernel to explicitly add calls to panic() or BUG() into
> code sites that introduce the taint flags of interest.
> For instance, if one is interested in following up with
> a post mortem analysis at the point a code path is hitting
> a bad page (i.e. unaccount_page_cache_page(), or slab_bug()),
> a crashdump could be collected by rebooting the kernel with
> 'panic_on_taint=0x20' amended to the command line string.
>
> Another, perhaps less frequent, use for this option would be
> as a mean for assuring a security policy case where only a
> subset of taints, or no single taint (in paranoid mode),
> is allowed for the running system.
> The optional switch 'nousertaint' is handy in this particular
> scenario as it will avoid userspace induced crashes by writes
> to /proc/sys/kernel/tainted causing false positive hits for
> such policies.
>
> Suggested-by: Qian Cai <cai@xxxxxx>
> Signed-off-by: Rafael Aquini <aquini@xxxxxxxxxx>
> ---
> Changelog:
> * v2: get rid of unnecessary/misguided compiler hints (Luis)
> enhance documentation text for the new kernel parameter (Randy)
> * v3: drop sysctl interface, keep it only as a kernel parameter (Luis)
> * v4: change panic_on_taint input from alphabetical taint flags
> to hexadecimal bitmasks, for clarity and extendability (Luis)
>
> Documentation/admin-guide/kdump/kdump.rst | 7 ++++
> .../admin-guide/kernel-parameters.txt | 13 +++++++
> include/linux/kernel.h | 4 +++
> kernel/panic.c | 34 +++++++++++++++++++
> kernel/sysctl.c | 11 +++++-
> 5 files changed, 68 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/admin-guide/kdump/kdump.rst b/Documentation/admin-guide/kdump/kdump.rst
> index ac7e131d2935..2707de840fd3 100644
> --- a/Documentation/admin-guide/kdump/kdump.rst
> +++ b/Documentation/admin-guide/kdump/kdump.rst
> @@ -521,6 +521,13 @@ will cause a kdump to occur at the panic() call. In cases where a user wants
> to specify this during runtime, /proc/sys/kernel/panic_on_warn can be set to 1
> to achieve the same behaviour.
>
> +Trigger Kdump on add_taint()
> +============================
> +
> +The kernel parameter panic_on_taint facilitates calling panic() from within
> +add_taint() whenever the value set in this bitmask matches with the bit flag
> +being set by add_taint(). This will cause a kdump to occur at the panic() call.
> +
> Contact
> =======
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 7bc83f3d9bdf..ce17fdbec7d1 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -3401,6 +3401,19 @@
> bit 4: print ftrace buffer
> bit 5: print all printk messages in buffer
>
> + panic_on_taint= Bitmask for conditionally call panic() in add_taint()
> + Format: <hex>[,nousertaint]
> + Hexadecimal bitmask representing the set of TAINT flags
> + that will cause the kernel to panic when add_taint() is
> + called with any of the flags in this set.
> + The optional switch "nousertaint" can be utilized to
> + prevent userland forced crashes by writing to sysctl
> + /proc/sys/kernel/tainted any flagset matching with the
> + bitmask set on panic_on_taint.
> + See Documentation/admin-guide/tainted-kernels.rst for
> + extra details on the taint flags that users can pick
> + to compose the bitmask to assign to panic_on_taint.
> +
> panic_on_warn panic() instead of WARN(). Useful to cause kdump
> on a WARN().
>
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index 9b7a8d74a9d6..70712944dffc 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -528,6 +528,8 @@ extern int panic_on_oops;
> extern int panic_on_unrecovered_nmi;
> extern int panic_on_io_nmi;
> extern int panic_on_warn;
> +extern unsigned long panic_on_taint;
> +extern bool panic_on_taint_nousertaint;
> extern int sysctl_panic_on_rcu_stall;
> extern int sysctl_panic_on_stackoverflow;
>
> @@ -597,6 +599,8 @@ extern enum system_states {
> #define TAINT_RANDSTRUCT 17
> #define TAINT_FLAGS_COUNT 18
>
> +#define TAINT_FLAGS_MAX ((1UL << TAINT_FLAGS_COUNT) - 1)
> +
> struct taint_flag {
> char c_true; /* character printed when tainted */
> char c_false; /* character printed when not tainted */
> diff --git a/kernel/panic.c b/kernel/panic.c
> index b69ee9e76cb2..94b5c973770c 100644
> --- a/kernel/panic.c
> +++ b/kernel/panic.c
> @@ -44,6 +44,8 @@ static int pause_on_oops_flag;
> static DEFINE_SPINLOCK(pause_on_oops_lock);
> bool crash_kexec_post_notifiers;
> int panic_on_warn __read_mostly;
> +unsigned long panic_on_taint;
> +bool panic_on_taint_nousertaint = false;
>
> int panic_timeout = CONFIG_PANIC_TIMEOUT;
> EXPORT_SYMBOL_GPL(panic_timeout);
> @@ -434,6 +436,11 @@ void add_taint(unsigned flag, enum lockdep_ok lockdep_ok)
> pr_warn("Disabling lock debugging due to kernel taint\n");
>
> set_bit(flag, &tainted_mask);
> +
> + if (tainted_mask & panic_on_taint) {
> + panic_on_taint = 0;
> + panic("panic_on_taint set ...");
> + }
> }
> EXPORT_SYMBOL(add_taint);
>
> @@ -686,3 +693,30 @@ static int __init oops_setup(char *s)
> return 0;
> }
> early_param("oops", oops_setup);
> +
> +static int __init panic_on_taint_setup(char *s)
> +{
> + char *taint_str;
> +
> + if (!s)
> + return -EINVAL;
> +
> + taint_str = strsep(&s, ",");
> + if (kstrtoul(taint_str, 16, &panic_on_taint))
> + return -EINVAL;
> +
> + /* make sure panic_on_taint doesn't hold out-of-range TAINT flags */
> + panic_on_taint &= TAINT_FLAGS_MAX;
While it may have made sennse for simplicity to not pr_warn_once on the
proc_taint() case I think in this case we do want to pr_warn_once() as
the user is wishing to DEFINITELY PANIC if such a taint flag is present.
> +
> + if (!panic_on_taint)
> + return -EINVAL;
> +
> + if (s && !strcmp(s, "nousertaint"))
> + panic_on_taint_nousertaint = true;
> +
> + pr_info("panic_on_taint: bitmask=0x%lx nousertaint_mode=%sabled\n",
> + panic_on_taint, panic_on_taint_nousertaint ? "en" : "dis");
> +
> + return 0;
> +}
> +early_param("panic_on_taint", panic_on_taint_setup);
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 8a176d8727a3..e257c965683a 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -2623,11 +2623,20 @@ static int proc_taint(struct ctl_table *table, int write,
> return err;
>
> if (write) {
> + int i;
> +
> + /*
> + * If we are relying on panic_on_taint not producing
> + * false positives due to userland input, bail out
> + * before setting the requested taint flags.
> + */
> + if (panic_on_taint_nousertaint && (tmptaint & panic_on_taint))
> + return -EINVAL;
> +
I like the compromise, but I think you also have to update this sysctl's
documentation to reflect this is disabled if this new boot param is used.
Luis
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