On Thu, 26 Sept 2024 at 19:02, Steven Rostedt <[email protected]> wrote: > > From: Steven Rostedt <[email protected]> > > At the 2024 Linux Plumbers Conference, I was talking with Hans de Goede > about the persistent buffer to display traces from previous boots. He > mentioned that UEFI can clear memory. In my own tests I have not seen > this. He later informed me that it requires the config option: > > CONFIG_RESET_ATTACK_MITIGATION > > It appears that setting this will allow the memory to be cleared on boot > up, which will definitely clear out the trace of the previous boot. > > Add this information under the trace_instance in kernel-parameters.txt > to let people know that this can cause issues. > > Link: > https://lore.kernel.org/all/[email protected]/ > > Reported-by: Hans de Goede <[email protected]> > Signed-off-by: Steven Rostedt (Google) <[email protected]> > --- > Documentation/admin-guide/kernel-parameters.txt | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/Documentation/admin-guide/kernel-parameters.txt > b/Documentation/admin-guide/kernel-parameters.txt > index bb48ae24ae69..f9b79294f84a 100644 > --- a/Documentation/admin-guide/kernel-parameters.txt > +++ b/Documentation/admin-guide/kernel-parameters.txt > @@ -6850,6 +6850,9 @@ > > reserve_mem=12M:4096:trace > trace_instance=boot_map^traceoff^traceprintk@trace,sched,irq > > + Note, CONFIG_RESET_ATTACK_MITIGATION can force a > memory reset on boot which > + will clear any trace that was stored. > +
CONFIG_RESET_ATTACK_MITIGATION can force a wipe of system RAM at warm reboot on systems that have a TPM enabled, but disabling it does not prevent it. Also, there are many other reasons why the trace buffer region may be wiped and/or reused for other purposes, so singling out CONFIG_RESET_ATTACK_MITIGATION like this is not that useful imo. As I indicated when this feature was under review, it should be made very clear that any kernel side changes that affect the system's behavior in this regard should not be considered regressions. So instead of mentioning CONFIG_RESET_ATTACK_MITIGATION here, perhaps it would be better to document that system firmware generally makes no guarantees about preserving memory contents, and so this feature may break without warning. Note that on UEFI systems, there is a so-called 'capsule' interface which does allow the OS to preserve an arbitrary buffer across a warm reboot. However, whether it works on all systems is anybody's guess.
