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SubjectRe: [PATCH] mm/mincore: allow for making sys_mincore() privileged
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On 5.1.2019 18:27, Jiri Kosina wrote:
> From: Jiri Kosina <jkosina@suse.cz>
>
> There are possibilities [1] how mincore() could be used as a converyor of
> a sidechannel information about pagecache metadata.
>
> Provide vm.mincore_privileged sysctl, which makes it possible to mincore()
> start returning -EPERM in case it's invoked by a process lacking
> CAP_SYS_ADMIN.

Haven't checked the details yet, but wouldn't it be safe if anonymous private
mincore() kept working, and restrictions were applied only to page cache?

> The default behavior stays "mincore() can be used by anybody" in order to
> be conservative with respect to userspace behavior.

What if we lied instead of returned -EPERM, to not break userspace so obviously?
I guess false positive would be the safer lie?

> [1] https://www.theregister.co.uk/2019/01/05/boffins_beat_page_cache/
>
> Signed-off-by: Jiri Kosina <jkosina@suse.cz>
> ---
> Documentation/sysctl/vm.txt | 9 +++++++++
> kernel/sysctl.c | 8 ++++++++
> mm/mincore.c | 5 +++++
> 3 files changed, 22 insertions(+)
>
> diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
> index 187ce4f599a2..afb8635e925e 100644
> --- a/Documentation/sysctl/vm.txt
> +++ b/Documentation/sysctl/vm.txt
> @@ -41,6 +41,7 @@ Currently, these files are in /proc/sys/vm:
> - min_free_kbytes
> - min_slab_ratio
> - min_unmapped_ratio
> +- mincore_privileged
> - mmap_min_addr
> - mmap_rnd_bits
> - mmap_rnd_compat_bits
> @@ -485,6 +486,14 @@ files and similar are considered.
> The default is 1 percent.
>
> ==============================================================
> +mincore_privileged:
> +
> +mincore() could be potentially used to mount a side-channel attack against
> +pagecache metadata. This sysctl provides system administrators means to
> +make it available only to processess that own CAP_SYS_ADMIN capability.
> +
> +The default is 0, which means mincore() can be used without restrictions.
> +==============================================================
>
> mmap_min_addr
>
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 1825f712e73b..f03cb07c8dd4 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -114,6 +114,7 @@ extern unsigned int sysctl_nr_open_min, sysctl_nr_open_max;
> #ifndef CONFIG_MMU
> extern int sysctl_nr_trim_pages;
> #endif
> +extern int sysctl_mincore_privileged;
>
> /* Constants used for minimum and maximum */
> #ifdef CONFIG_LOCKUP_DETECTOR
> @@ -1684,6 +1685,13 @@ static struct ctl_table vm_table[] = {
> .extra2 = (void *)&mmap_rnd_compat_bits_max,
> },
> #endif
> + {
> + .procname = "mincore_privileged",
> + .data = &sysctl_mincore_privileged,
> + .maxlen = sizeof(sysctl_mincore_privileged),
> + .mode = 0644,
> + .proc_handler = proc_dointvec,
> + },
> { }
> };
>
> diff --git a/mm/mincore.c b/mm/mincore.c
> index 218099b5ed31..77d4928cdfaa 100644
> --- a/mm/mincore.c
> +++ b/mm/mincore.c
> @@ -21,6 +21,8 @@
> #include <linux/uaccess.h>
> #include <asm/pgtable.h>
>
> +int sysctl_mincore_privileged;
> +
> static int mincore_hugetlb(pte_t *pte, unsigned long hmask, unsigned long addr,
> unsigned long end, struct mm_walk *walk)
> {
> @@ -228,6 +230,9 @@ SYSCALL_DEFINE3(mincore, unsigned long, start, size_t, len,
> unsigned long pages;
> unsigned char *tmp;
>
> + if (sysctl_mincore_privileged && !capable(CAP_SYS_ADMIN))
> + return -EPERM;
> +
> /* Check the start address: needs to be page-aligned.. */
> if (start & ~PAGE_MASK)
> return -EINVAL;
>

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