This simple Kconfig option removes the FOLL_FORCE flag from procfs write calls because it can be abused. Enabling it breaks some debuggers like GDB so it defaults off. Previously we tried a more sophisticated approach allowing distributions to fine-tune proc/pid/mem behaviour via both kconfig and boot params, however that got NAK-ed by Linus [1] who prefers this simpler approach. 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: Adrian Ratiu <adrian.ratiu@xxxxxxxxxxxxx> --- fs/proc/base.c | 6 +++++- security/Kconfig | 14 ++++++++++++++ 2 files changed, 19 insertions(+), 1 deletion(-) diff --git a/fs/proc/base.c b/fs/proc/base.c index 72a1acd03675..53ad71d7d785 100644 --- a/fs/proc/base.c +++ b/fs/proc/base.c @@ -855,7 +855,11 @@ 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); + +#ifndef CONFIG_SECURITY_PROC_MEM_RESTRICT_FOLL_FORCE + flags |= FOLL_FORCE; +#endif while (count > 0) { size_t this_len = min_t(size_t, count, PAGE_SIZE); diff --git a/security/Kconfig b/security/Kconfig index 412e76f1575d..24053b8ff786 100644 --- a/security/Kconfig +++ b/security/Kconfig @@ -19,6 +19,20 @@ config SECURITY_DMESG_RESTRICT If you are unsure how to answer this question, answer N. +config SECURITY_PROC_MEM_RESTRICT_FOLL_FORCE + bool "Remove FOLL_FORCE usage from /proc/pid/mem writes" + default n + help + This restricts FOLL_FORCE flag usage in procfs mem write calls + because it bypasses memory permission checks and can be used by + attackers to manipulate process memory contents that would be + otherwise protected. + + Enabling this will break GDB, gdbserver and other debuggers + which require FOLL_FORCE for basic functionalities. + + If you are unsure how to answer this question, answer N. + config SECURITY bool "Enable different security models" depends on SYSFS -- 2.44.2
