On Mon, Sep 20, 2021 at 11:23:01AM -0700, Stephen Boyd wrote: > Quoting Kees Cook (2021-09-20 07:29:54) > > On Tue, Jun 01, 2021 at 11:22:02AM -0700, Stephen Boyd wrote: > > > Obscuring the pointers that slub shows when debugging makes for some > > > confusing slub debug messages: > > > > > > Padding overwritten. 0x0000000079f0674a-0x000000000d4dce17 > > > > > > Those addresses are hashed for kernel security reasons. If we're trying > > > to be secure with slub_debug on the commandline we have some big > > > problems given that we dump whole chunks of kernel memory to the kernel > > > logs. Let's force on the no_hash_pointers commandline flag when > > > slub_debug is on the commandline. This makes slub debug messages more > > > meaningful and if by chance a kernel address is in some slub debug > > > object dump we will have a better chance of figuring out what went > > > wrong. > > > > > > Note that we don't use %px in the slub code because we want to reduce > > > the number of places that %px is used in the kernel. This also nicely > > > prints a big fat warning at kernel boot if slub_debug is on the > > > commandline so that we know that this kernel shouldn't be used on > > > production systems. > > > > Eeeek. I missed this patch. NAK NAK. People use slub_debug for > > production systems to gain redzoning, etc, as a layer of defense, and > > they absolutely do not want %p-hashing disabled. %p hashing is > > controlled by the no_hash_pointers boot param (since v5.12), and needs to stay > > separate from slub_debug. > > > > Can we please revert this in Linus's tree and in v5.14? > > > > This is fine with me as long as debugging with slub_debug on the > commandline is possible. Would you prefer v1 of this patch series[1] > that uses the printk format to print unhashed pointers in slub debugging > messages? > > [1] https://lore.kernel.org/r/20210520013539.3733631-1-swboyd@xxxxxxxxxxxx I'd like to keep %px use in the kernel minimized. Seeing full pointers (%p hashing disabled) can be done with the no_hash_pointers boot param, and that's used in other debug cases as well. I'd rather keep it a global knob. Thanks! -Kees -- Kees Cook
