[PATCH v4] proc: add config & param to block forcing mem writes
Jeff Xu
jeffxu at google.com
Tue Jul 30 23:08:35 UTC 2024
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.
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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