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Subject[PATCH] mm/mincore: allow for making sys_mincore() privileged
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.

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

[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;
--
Jiri Kosina
SUSE Labs
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