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_2vta0nlzcxorgkue5...@mail.gmail.com/ [1] Cc: Doug Anderson <[email protected]> Cc: Jeff Xu <[email protected]> Cc: Jann Horn <[email protected]> Cc: Kees Cook <[email protected]> Cc: Ard Biesheuvel <[email protected]> Cc: Christian Brauner <[email protected]> Suggested-by: Linus Torvalds <[email protected]> Signed-off-by: Linus Torvalds <[email protected]> Signed-off-by: Adrian Ratiu <[email protected]> --- 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); + + 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; + 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
