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 <diand...@chromium.org> Cc: Jeff Xu <jef...@google.com> Cc: Jann Horn <ja...@google.com> Cc: Kees Cook <k...@kernel.org> Cc: Christian Brauner <brau...@kernel.org> Suggested-by: Linus Torvalds <torva...@linux-foundation.org> Signed-off-by: Linus Torvalds <torva...@linux-foundation.org> Signed-off-by: Adrian Ratiu <adrian.ra...@collabora.com> --- .../admin-guide/kernel-parameters.txt | 10 ++++ fs/proc/base.c | 58 ++++++++++++++++++- security/Kconfig | 32 ++++++++++ 3 files changed, 99 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. + Can be one of: + - 'always' traditional behavior always allows mem overrides. + - 'ptrace' only allow for active ptracers. + - 'never' never allow mem permission overrides. + If not specified, default is 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..5ef14ba953a2 100644 --- a/fs/proc/base.c +++ b/fs/proc/base.c @@ -117,6 +117,40 @@ static u8 nlink_tid __ro_after_init; static u8 nlink_tgid __ro_after_init; +enum proc_mem_force_state { + PROC_MEM_FORCE_ALWAYS, + PROC_MEM_FORCE_PTRACE, + PROC_MEM_FORCE_NEVER +}; + +#if defined(CONFIG_PROC_MEM_ALWAYS_FORCE) +static enum proc_mem_force_state proc_mem_force_override __ro_after_init = PROC_MEM_FORCE_ALWAYS; +#elif defined(CONFIG_PROC_MEM_FORCE_PTRACE) +static enum proc_mem_force_state proc_mem_force_override __ro_after_init = PROC_MEM_FORCE_PTRACE; +#else +static enum proc_mem_force_state proc_mem_force_override __ro_after_init = PROC_MEM_FORCE_NEVER; +#endif + +static int __init early_proc_mem_force_override(char *buf) +{ + if (!buf) + return -EINVAL; + + if (strcmp(buf, "always") == 0) { + proc_mem_force_override = PROC_MEM_FORCE_ALWAYS; + } else if (strcmp(buf, "ptrace") == 0) { + proc_mem_force_override = PROC_MEM_FORCE_PTRACE; + } else if (strcmp(buf, "never") == 0) { + proc_mem_force_override = PROC_MEM_FORCE_NEVER; + } else { + pr_warn("proc_mem.force_override: ignoring unknown option '%s'\n", buf); + return -EINVAL; + } + + return 0; +} +early_param("proc_mem.force_override", early_proc_mem_force_override); + struct pid_entry { const char *name; unsigned int len; @@ -835,6 +869,26 @@ 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) +{ + switch (proc_mem_force_override) { + case PROC_MEM_FORCE_NEVER: + return false; + case PROC_MEM_FORCE_PTRACE: { + bool ptrace_active = false; + struct task_struct *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 +909,9 @@ 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
