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Date: Thu, 11 Dec 2014 14:21:45 -0800 From: Kees Cook <keescook@...omium.org> To: linux-kernel@...r.kernel.org Cc: Rusty Russell <rusty@...tcorp.com.au>, Christoph Hellwig <hch@...radead.org>, Jani Nikula <jani.nikula@...el.com>, Hannes Reinecke <hare@...e.de>, Geert Uytterhoeven <geert@...ux-m68k.org> Subject: [PATCH] param: do not set store func without write perm When a module_param is defined without DAC write permissions, it can still be changed at runtime and updated. Drivers using a 0444 permission may be surprised that these values can still be changed. For drivers that want to allow updates, any S_IW* flag will set the "store" function as before. Drivers without S_IW* flags will have the "store" function unset, unforcing a read-only value. Drivers that wish neither "store" nor "get" can continue to use "0" for perms to stay out of sysfs entirely. Old behavior: # cd /sys/module/snd/parameters # ls -l total 0 -r--r--r-- 1 root root 4096 Dec 11 13:55 cards_limit -r--r--r-- 1 root root 4096 Dec 11 13:55 major -r--r--r-- 1 root root 4096 Dec 11 13:55 slots # cat major 116 # echo -1 > major -bash: major: Permission denied # chmod u+w major # echo -1 > major # cat major -1 New behavior: ... # chmod u+w major # echo -1 > major -bash: echo: write error: Input/output error Signed-off-by: Kees Cook <keescook@...omium.org> --- kernel/params.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/kernel/params.c b/kernel/params.c index db97b791390f..fd50ce9f1bbf 100644 --- a/kernel/params.c +++ b/kernel/params.c @@ -647,7 +647,9 @@ static __modinit int add_sysfs_param(struct module_kobject *mk, sysfs_attr_init(&new->attrs[num].mattr.attr); new->attrs[num].param = kp; new->attrs[num].mattr.show = param_attr_show; - new->attrs[num].mattr.store = param_attr_store; + /* Do not allow runtime DAC changes to make param writable. */ + if ((kp->perm & (S_IWUSR | S_IWGRP | S_IWOTH)) != 0) + new->attrs[num].mattr.store = param_attr_store; new->attrs[num].mattr.attr.name = (char *)name; new->attrs[num].mattr.attr.mode = kp->perm; new->num = num+1; -- 1.9.1 -- Kees Cook Chrome OS Security -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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