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Date: Tue, 12 May 2015 12:22:27 +0100
From: Mark Williamson <mwilliamson@...o-software.com>
To: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Cc: linux-mm@...ck.org, Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
kernel list <linux-kernel@...r.kernel.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Pavel Emelyanov <xemul@...allels.com>,
Linux API <linux-api@...r.kernel.org>,
Andy Lutomirski <luto@...capital.net>,
Vlastimil Babka <vbabka@...e.cz>, Pavel Machek <pavel@....cz>,
Mark Seaborn <mseaborn@...omium.org>,
"Kirill A. Shutemov" <kirill@...temov.name>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Daniel James <djames@...o-software.com>,
Finn Grimwood <fgrimwood@...o-software.com>
Subject: Re: [PATCH v2 2/3] pagemap: hide physical addresses from
non-privileged users
Hi Konstantin,
Comments inline...
On Tue, May 12, 2015 at 10:43 AM, Konstantin Khlebnikov
<khlebnikov@...dex-team.ru> wrote:
> This patch makes pagemap readable for normal users back but hides physical
> addresses from them. For some use cases PFN isn't required at all: flags
> give information about presence, page type (anon/file/swap), soft-dirty mark,
> and hint about page mapcount state: exclusive(mapcount = 1) or (mapcount > 1).
>
> Signed-off-by: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
> Fixes: ab676b7d6fbf ("pagemap: do not leak physical addresses to non-privileged userspace")
> Link: lkml.kernel.org/r/1425935472-17949-1-git-send-email-kirill@...temov.name
> ---
> fs/proc/task_mmu.c | 36 ++++++++++++++++++++++--------------
> 1 file changed, 22 insertions(+), 14 deletions(-)
>
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 29febec65de4..0b7a8ffec95f 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -962,6 +962,7 @@ struct pagemapread {
> int pos, len; /* units: PM_ENTRY_BYTES, not bytes */
> pagemap_entry_t *buffer;
> bool v2;
> + bool show_pfn;
> };
>
> #define PAGEMAP_WALK_SIZE (PMD_SIZE)
> @@ -1046,12 +1047,13 @@ out:
> static void pte_to_pagemap_entry(pagemap_entry_t *pme, struct pagemapread *pm,
> struct vm_area_struct *vma, unsigned long addr, pte_t pte)
> {
> - u64 frame, flags;
> + u64 frame = 0, flags;
> struct page *page = NULL;
> int flags2 = 0;
>
> if (pte_present(pte)) {
> - frame = pte_pfn(pte);
> + if (pm->show_pfn)
> + frame = pte_pfn(pte);
> flags = PM_PRESENT;
> page = vm_normal_page(vma, addr, pte);
> if (pte_soft_dirty(pte))
> @@ -1087,15 +1089,19 @@ static void pte_to_pagemap_entry(pagemap_entry_t *pme, struct pagemapread *pm,
> static void thp_pmd_to_pagemap_entry(pagemap_entry_t *pme, struct pagemapread *pm,
> pmd_t pmd, int offset, int pmd_flags2)
> {
> + u64 frame = 0;
> +
> /*
> * Currently pmd for thp is always present because thp can not be
> * swapped-out, migrated, or HWPOISONed (split in such cases instead.)
> * This if-check is just to prepare for future implementation.
> */
> - if (pmd_present(pmd))
> - *pme = make_pme(PM_PFRAME(pmd_pfn(pmd) + offset)
> - | PM_STATUS2(pm->v2, pmd_flags2) | PM_PRESENT);
> - else
> + if (pmd_present(pmd)) {
> + if (pm->show_pfn)
> + frame = pmd_pfn(pmd) + offset;
> + *pme = make_pme(PM_PFRAME(frame) | PM_PRESENT |
> + PM_STATUS2(pm->v2, pmd_flags2));
> + } else
> *pme = make_pme(PM_NOT_PRESENT(pm->v2) | PM_STATUS2(pm->v2, pmd_flags2));
> }
> #else
> @@ -1171,11 +1177,14 @@ static int pagemap_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
> static void huge_pte_to_pagemap_entry(pagemap_entry_t *pme, struct pagemapread *pm,
> pte_t pte, int offset, int flags2)
> {
> - if (pte_present(pte))
> - *pme = make_pme(PM_PFRAME(pte_pfn(pte) + offset) |
> - PM_STATUS2(pm->v2, flags2) |
> - PM_PRESENT);
> - else
> + u64 frame = 0;
> +
> + if (pte_present(pte)) {
> + if (pm->show_pfn)
> + frame = pte_pfn(pte) + offset;
> + *pme = make_pme(PM_PFRAME(frame) | PM_PRESENT |
> + PM_STATUS2(pm->v2, flags2));
> + } else
> *pme = make_pme(PM_NOT_PRESENT(pm->v2) |
> PM_STATUS2(pm->v2, flags2));
> }
> @@ -1260,6 +1269,8 @@ static ssize_t pagemap_read(struct file *file, char __user *buf,
> if (!count)
> goto out_task;
>
> + /* do not disclose physical addresses: attack vector */
> + pm.show_pfn = capable(CAP_SYS_ADMIN);
If I understood correctly, Linus recommended to me that we use the
open-time capabilities of the file descriptor rather than the current
capability state (to mitigate against an attacker passing an FD to a
setuid process, I think).
FWIW, I knocked up a quick internal patch (less comprehensive than
yours!) and used file_ns_capable() successfully, i.e:
pm.show_pfn = file_ns_capable(file, &init_user_ns, CAP_SYS_ADMIN);
It looked promising to be but I've not done the checking to verify
that this is strictly correct; the capabilities stuff is not an area
of the kernel I'm familiar with.
> pm.v2 = soft_dirty_cleared;
> pm.len = (PAGEMAP_WALK_SIZE >> PAGE_SHIFT);
> pm.buffer = kmalloc(pm.len * PM_ENTRY_BYTES, GFP_TEMPORARY);
> @@ -1335,9 +1346,6 @@ out:
>
> static int pagemap_open(struct inode *inode, struct file *file)
> {
> - /* do not disclose physical addresses: attack vector */
> - if (!capable(CAP_SYS_ADMIN))
> - return -EPERM;
> pr_warn_once("Bits 55-60 of /proc/PID/pagemap entries are about "
> "to stop being page-shift some time soon. See the "
> "linux/Documentation/vm/pagemap.txt for details.\n");
>
No other comments on this, looks like it would help us.
Thanks,
Mark
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