On Friday, July 26, 2024 13:18 EEST, Ard Biesheuvel <ardb@xxxxxxxxxx> wrote: > On Fri, 26 Jul 2024 at 11:11, Adrian Ratiu <adrian.ratiu@xxxxxxxxxxxxx> wrote: > > > > This adds a Kconfig option and boot param to allow removing > > the FOLL_FORCE flag from /proc/pid/mem write calls because > > it can be abused. > > > > The traditional forcing behavior is kept as default because > > it can break GDB and some other use cases. > > > > Previously we tried a more sophisticated approach allowing > > distributions to fine-tune /proc/pid/mem behavior, however > > that got NAK-ed by Linus [1], who prefers this simpler > > approach with semantics also easier to understand for users. > > > > Link: https://lore.kernel.org/lkml/CAHk-=wiGWLChxYmUA5HrT5aopZrB7_2VTa0NLZcxORgkUe5tEQ@xxxxxxxxxxxxxx/ [1] > > Cc: Doug Anderson <dianders@xxxxxxxxxxxx> > > Cc: Jeff Xu <jeffxu@xxxxxxxxxx> > > Cc: Jann Horn <jannh@xxxxxxxxxx> > > Cc: Kees Cook <kees@xxxxxxxxxx> > > Cc: Christian Brauner <brauner@xxxxxxxxxx> > > Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> > > Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> > > Signed-off-by: Adrian Ratiu <adrian.ratiu@xxxxxxxxxxxxx> > > --- > > Changes in v3: > > * Simplified code to use shorthand ifs and a > > lookup_constant() table. > > > > Changes in v2: > > * Added bootparam on top of Linus' patch. > > * Slightly reworded commit msg. > > --- > > .../admin-guide/kernel-parameters.txt | 10 ++++ > > fs/proc/base.c | 54 ++++++++++++++++++- > > security/Kconfig | 32 +++++++++++ > > 3 files changed, 95 insertions(+), 1 deletion(-) > > > > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt > > index c1134ad5f06d..793301f360ec 100644 > > --- a/Documentation/admin-guide/kernel-parameters.txt > > +++ b/Documentation/admin-guide/kernel-parameters.txt > > @@ -4791,6 +4791,16 @@ > > printk.time= Show timing data prefixed to each printk message line > > Format: <bool> (1/Y/y=enable, 0/N/n=disable) > > > > + proc_mem.force_override= [KNL] > > + Format: {always | ptrace | never} > > + Traditionally /proc/pid/mem allows users to override memory > > + permissions. This allows people to limit that. > > Better to use passive tense here rather than referring to 'users' and 'people'. > > 'Traditionally, /proc/pid/mem allows memory permissions to be > overridden without restrictions. > This option may be set to restrict that' > > > + Can be one of: > > + - 'always' traditional behavior always allows mem overrides. > > punctuation please > > > + - 'ptrace' only allow for active ptracers. > > + - 'never' never allow mem permission overrides. > > Please be consistent: 'mem overrides' or 'mem permission overrides' in > both instances. > > > + If not specified, default is always. > > 'always' > > > + > > processor.max_cstate= [HW,ACPI] > > Limit processor to maximum C-state > > max_cstate=9 overrides any DMI blacklist limit. > > diff --git a/fs/proc/base.c b/fs/proc/base.c > > index 72a1acd03675..0ca3fc3d9e0e 100644 > > --- a/fs/proc/base.c > > +++ b/fs/proc/base.c > > @@ -85,6 +85,7 @@ > > #include <linux/elf.h> > > #include <linux/pid_namespace.h> > > #include <linux/user_namespace.h> > > +#include <linux/fs_parser.h> > > #include <linux/fs_struct.h> > > #include <linux/slab.h> > > #include <linux/sched/autogroup.h> > > @@ -117,6 +118,35 @@ > > static u8 nlink_tid __ro_after_init; > > static u8 nlink_tgid __ro_after_init; > > > > +enum proc_mem_force { > > + PROC_MEM_FORCE_ALWAYS, > > + PROC_MEM_FORCE_PTRACE, > > + PROC_MEM_FORCE_NEVER > > +}; > > + > > +static enum proc_mem_force proc_mem_force_override __ro_after_init = > > + IS_ENABLED(CONFIG_PROC_MEM_ALWAYS_FORCE) ? PROC_MEM_FORCE_ALWAYS : > > + IS_ENABLED(CONFIG_PROC_MEM_FORCE_PTRACE) ? PROC_MEM_FORCE_PTRACE : > > + PROC_MEM_FORCE_NEVER; > > + > > +struct constant_table proc_mem_force_table[] = { > > This can be static const __initconst > > > + { "always", PROC_MEM_FORCE_ALWAYS }, > > + { "ptrace", PROC_MEM_FORCE_PTRACE }, > > + { } > > +}; > > + > > +static int __init early_proc_mem_force_override(char *buf) > > +{ > > + if (!buf) > > + return -EINVAL; > > + > > Can this ever happen? Not sure, many calls simply ignore this case while others like this [1] printk example do test it. I'm inclined to think it can't happen however it's still to good to error check. Thanks for all the suggestions, I'll leave this a bit for others to get a chance to review, then send another iteration. [1] https://elixir.bootlin.com/linux/v6.10.1/source/kernel/printk/printk.c#L1051
