Re: [PATCH v4] proc: add config & param to block forcing mem writes
Adrian Ratiu
adrian.ratiu at collabora.com
Wed Jul 31 13:48:38 UTC 2024
On Wednesday, July 31, 2024 02:08 EEST, Jeff Xu <jeffxu at google.com> wrote:
> On Tue, Jul 30, 2024 at 6:25 AM Adrian Ratiu <adrian.ratiu at collabora.com> 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@mail.gmail.com/ [1]
> > Cc: Doug Anderson <dianders at chromium.org>
> > Cc: Jeff Xu <jeffxu at google.com>
> > Cc: Jann Horn <jannh at google.com>
> > Cc: Kees Cook <kees at kernel.org>
> > Cc: Ard Biesheuvel <ardb at kernel.org>
> > Cc: Christian Brauner <brauner at kernel.org>
> > Suggested-by: Linus Torvalds <torvalds at linux-foundation.org>
> > Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
> > Signed-off-by: Adrian Ratiu <adrian.ratiu at collabora.com>
> > ---
> > Changes in v4:
> > * Fixed doc punctuation, used passive tense, improved
> > wording consistency, fixed default value wording
> > * Made struct constant_table a static const __initconst
> > * Reworked proc_mem_foll_force() indentation and var
> > declarations to make code clearer
> > * Reworked enum + struct definition so lookup_constant()
> > defaults to 'always'.
> >
> > 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 f1384c7b59c9..8396e015aab3 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -4788,6 +4788,16 @@
> > 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 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.
> > + - 'ptrace': only allow mem overrides for active ptracers.
> > + - 'never': never allow mem overrides.
> > + If not specified, default is the CONFIG_PROC_MEM_* choice.
> > +
> > 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..daacb8070042 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_NO_FORCE) ? PROC_MEM_FORCE_NEVER :
> > + IS_ENABLED(CONFIG_PROC_MEM_FORCE_PTRACE) ? PROC_MEM_FORCE_PTRACE :
> > + PROC_MEM_FORCE_ALWAYS;
> > +
> > +static const struct constant_table proc_mem_force_table[] __initconst = {
> > + { "never", PROC_MEM_FORCE_NEVER },
> > + { "ptrace", PROC_MEM_FORCE_PTRACE },
> > + { }
> > +};
> > +
> > +static int __init early_proc_mem_force_override(char *buf)
> > +{
> > + if (!buf)
> > + return -EINVAL;
> > +
> > + proc_mem_force_override = lookup_constant(proc_mem_force_table,
> > + buf, PROC_MEM_FORCE_ALWAYS);
> proc_mem_force_table has two entries, this means:
> if kernel cmdline has proc_mem.force_override="invalid",
> PROC_MEM_FORCE_ALWAYS will be used.
>
> Another option is to have 3 entries in proc_mem_force_table: adding
> {"aways", PROC_MEM_FORCE_ALWAYS}
>
> and let lookup_constant return -1 when not found, and not override
> proc_mem_force_override.
Thanks Jeff for spotting this! :)
In addition to adding all the 3 entries as you suggested, I think we
can also do the following:
proc_mem_force_override = lookup_constant(proc_mem_force_table,
buf, proc_mem_force_override);
This will ensure that if something like "invalid" gets passed, the
proc_mem_force_override value remains unchanged. In other words
it remains equal to the default choice set via Kconfig and correctly
matches the doc description.
I'll address this before sending v5 in a few days to give others time
to review.
>
> This enforces the kernel cmd line must be set to one of three choices
> "always|ptrace|never" to be effective.
>
> If you choose this path: please modify kernel-parameters.txt to
> "If not specified or invalid, default is the CONFIG_PROC_MEM_* choice."
>
> or else please clarify in the kernel-parameters.text:
> If not specified, default is the CONFIG_PROC_MEM_* choice
> If invalid str or empty string, PROC_MEM_FORCE_ALWAYS will be used
> regardless CONFIG_PROC_MEM_* choice
>
> > +
> > + return 0;
> > +}
> > +early_param("proc_mem.force_override", early_proc_mem_force_override);
> > +
> > struct pid_entry {
> > const char *name;
> > unsigned int len;
> > @@ -835,6 +865,26 @@ static int mem_open(struct inode *inode, struct file *file)
> > return ret;
> > }
> >
> > +static bool proc_mem_foll_force(struct file *file, struct mm_struct *mm)
> > +{
> > + struct task_struct *task;
> > + bool ptrace_active = false;
> > +
> > + switch (proc_mem_force_override) {
> > + case PROC_MEM_FORCE_NEVER:
> > + return false;
> > + case PROC_MEM_FORCE_PTRACE:
> > + task = get_proc_task(file_inode(file));
> > + if (task) {
> > + ptrace_active = task->ptrace && task->mm == mm && task->parent == current;
> Do we need to call "read_lock(&tasklist_lock);" ?
> see comments in ptrace_check_attach() of kernel/ptrace.c
>
>
>
> > + put_task_struct(task);
> > + }
> > + return ptrace_active;
> > + default:
> > + return true;
> > + }
> > +}
> > +
> > static ssize_t mem_rw(struct file *file, char __user *buf,
> > size_t count, loff_t *ppos, int write)
> > {
> > @@ -855,7 +905,9 @@ static ssize_t mem_rw(struct file *file, char __user *buf,
> > if (!mmget_not_zero(mm))
> > goto free;
> >
> > - flags = FOLL_FORCE | (write ? FOLL_WRITE : 0);
> > + flags = write ? FOLL_WRITE : 0;
> > + if (proc_mem_foll_force(file, mm))
> > + flags |= FOLL_FORCE;
> >
> > while (count > 0) {
> > size_t this_len = min_t(size_t, count, PAGE_SIZE);
> > diff --git a/security/Kconfig b/security/Kconfig
> > index 412e76f1575d..a93c1a9b7c28 100644
> > --- a/security/Kconfig
> > +++ b/security/Kconfig
> > @@ -19,6 +19,38 @@ config SECURITY_DMESG_RESTRICT
> >
> > If you are unsure how to answer this question, answer N.
> >
> > +choice
> > + prompt "Allow /proc/pid/mem access override"
> > + default PROC_MEM_ALWAYS_FORCE
> > + help
> > + Traditionally /proc/pid/mem allows users to override memory
> > + permissions for users like ptrace, assuming they have ptrace
> > + capability.
> > +
> > + This allows people to limit that - either never override, or
> > + require actual active ptrace attachment.
> > +
> > + Defaults to the traditional behavior (for now)
> > +
> > +config PROC_MEM_ALWAYS_FORCE
> > + bool "Traditional /proc/pid/mem behavior"
> > + help
> > + This allows /proc/pid/mem accesses to override memory mapping
> > + permissions if you have ptrace access rights.
> > +
> > +config PROC_MEM_FORCE_PTRACE
> > + bool "Require active ptrace() use for access override"
> > + help
> > + This allows /proc/pid/mem accesses to override memory mapping
> > + permissions for active ptracers like gdb.
> > +
> > +config PROC_MEM_NO_FORCE
> > + bool "Never"
> > + help
> > + Never override memory mapping permissions
> > +
> > +endchoice
> > +
> > config SECURITY
> > bool "Enable different security models"
> > depends on SYSFS
> > --
> > 2.44.2
> >
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