[PATCH v3] x86/mm: warn on W+x mappings
From: Stephen Smalley
Date: Mon Oct 05 2015 - 12:57:03 EST
Warn on any residual W+x mappings after setting NX
if DEBUG_WX is enabled. Introduce a separate
X86_PTDUMP_CORE config that enables the code for
dumping the page tables without enabling the debugfs
interface, so that DEBUG_WX can be enabled without
exposing the debugfs interface. Switch EFI_PGT_DUMP
to using X86_PTDUMP_CORE so that it also does not require
enabling the debugfs interface.
On success:
x86/mm: Checked W+X mappings: passed, no W+X pages found.
On failure:
------------[ cut here ]------------
WARNING: CPU: 1 PID: 1 at arch/x86/mm/dump_pagetables.c:226 note_page+0x610/0x7b0()
x86/mm: Found insecure W+X mapping at address ffffffff81755000/__stop___ex_table+0xfa8/0xabfa8
Modules linked in:
CPU: 1 PID: 1 Comm: swapper/0 Tainted: G W 4.3.0-rc3+ #19
0000000000000000 00000000e96b193f ffff88042c5dbd48 ffffffff81380a5f
ffff88042c5dbd90 ffff88042c5dbd80 ffffffff8109d3f2 80000000000001e1
0000000000000003 ffff88042c5dbe90 ffff88042c5dbe90 0000000000000000
Call Trace:
[<ffffffff81380a5f>] dump_stack+0x44/0x55
[<ffffffff8109d3f2>] warn_slowpath_common+0x82/0xc0
[<ffffffff8109d48c>] warn_slowpath_fmt+0x5c/0x80
[<ffffffff8106cfc9>] ? note_page+0x5c9/0x7b0
[<ffffffff8106d010>] note_page+0x610/0x7b0
[<ffffffff8106d409>] ptdump_walk_pgd_level_core+0x259/0x3c0
[<ffffffff8106d5a7>] ptdump_walk_pgd_level_checkwx+0x17/0x20
[<ffffffff81063905>] mark_rodata_ro+0xf5/0x100
[<ffffffff817415a0>] ? rest_init+0x80/0x80
[<ffffffff817415bd>] kernel_init+0x1d/0xe0
[<ffffffff8174cd1f>] ret_from_fork+0x3f/0x70
[<ffffffff817415a0>] ? rest_init+0x80/0x80
---[ end trace a1f23a1e42a2ac76 ]---
x86/mm: Checked W+X mappings: FAILED, 171 W+X pages found.
Signed-off-by: Stephen Smalley <sds@xxxxxxxxxxxxx>
---
v3 enables the checks on 32-bit if NX is supported, and also
makes DEBUG_WX depend on DEBUG_RODATA since both the NX marking
and the checking occurs from mark_rodata_ro().
arch/x86/Kconfig.debug | 20 +++++++++++++++++++-
arch/x86/include/asm/pgtable.h | 7 +++++++
arch/x86/mm/Makefile | 2 +-
arch/x86/mm/dump_pagetables.c | 42 +++++++++++++++++++++++++++++++++++++++++-
arch/x86/mm/init_32.c | 2 ++
arch/x86/mm/init_64.c | 2 ++
6 files changed, 72 insertions(+), 3 deletions(-)
diff --git a/arch/x86/Kconfig.debug b/arch/x86/Kconfig.debug
index d8c0d32..d09fde7 100644
--- a/arch/x86/Kconfig.debug
+++ b/arch/x86/Kconfig.debug
@@ -65,10 +65,14 @@ config EARLY_PRINTK_EFI
This is useful for kernel debugging when your machine crashes very
early before the console code is initialized.
+config X86_PTDUMP_CORE
+ def_bool n
+
config X86_PTDUMP
bool "Export kernel pagetable layout to userspace via debugfs"
depends on DEBUG_KERNEL
select DEBUG_FS
+ select X86_PTDUMP_CORE
---help---
Say Y here if you want to show the kernel pagetable layout in a
debugfs file. This information is only useful for kernel developers
@@ -79,7 +83,8 @@ config X86_PTDUMP
config EFI_PGT_DUMP
bool "Dump the EFI pagetable"
- depends on EFI && X86_PTDUMP
+ depends on EFI
+ select X86_PTDUMP_CORE
---help---
Enable this if you want to dump the EFI page table before
enabling virtual mode. This can be used to debug miscellaneous
@@ -105,6 +110,19 @@ config DEBUG_RODATA_TEST
feature as well as for the change_page_attr() infrastructure.
If in doubt, say "N"
+config DEBUG_WX
+ bool "Warn on W+X mappings at boot"
+ depends on DEBUG_RODATA
+ select X86_PTDUMP_CORE
+ ---help---
+ Generate a warning if any W+X mappings are found at boot.
+ This is useful for discovering cases where the kernel is leaving
+ W+X mappings after applying NX, as such mappings are a security risk.
+ Look for a message in dmesg output like this:
+ x86/mm: Checked W+X mappings: passed, no W+X pages found.
+ or like this:
+ x86/mm: Checked W+X mappings: FAILED, <N> W+X pages found.
+
config DEBUG_SET_MODULE_RONX
bool "Set loadable kernel module data as NX and text as RO"
depends on MODULES
diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index 867da5b..f2b6bed 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -19,6 +19,13 @@
#include <asm/x86_init.h>
void ptdump_walk_pgd_level(struct seq_file *m, pgd_t *pgd);
+void ptdump_walk_pgd_level_checkwx(void);
+
+#ifdef CONFIG_DEBUG_WX
+#define debug_checkwx() ptdump_walk_pgd_level_checkwx()
+#else
+#define debug_checkwx() do { } while (0)
+#endif
/*
* ZERO_PAGE is a global shared page that is always zero: used
diff --git a/arch/x86/mm/Makefile b/arch/x86/mm/Makefile
index a482d10..65c47fd 100644
--- a/arch/x86/mm/Makefile
+++ b/arch/x86/mm/Makefile
@@ -14,7 +14,7 @@ obj-$(CONFIG_SMP) += tlb.o
obj-$(CONFIG_X86_32) += pgtable_32.o iomap_32.o
obj-$(CONFIG_HUGETLB_PAGE) += hugetlbpage.o
-obj-$(CONFIG_X86_PTDUMP) += dump_pagetables.o
+obj-$(CONFIG_X86_PTDUMP_CORE) += dump_pagetables.o
obj-$(CONFIG_HIGHMEM) += highmem_32.o
diff --git a/arch/x86/mm/dump_pagetables.c b/arch/x86/mm/dump_pagetables.c
index f0cedf3..19c64af 100644
--- a/arch/x86/mm/dump_pagetables.c
+++ b/arch/x86/mm/dump_pagetables.c
@@ -32,6 +32,8 @@ struct pg_state {
const struct addr_marker *marker;
unsigned long lines;
bool to_dmesg;
+ bool check_wx;
+ unsigned long wx_pages;
};
struct addr_marker {
@@ -214,6 +216,16 @@ static void note_page(struct seq_file *m, struct pg_state *st,
const char *unit = units;
unsigned long delta;
int width = sizeof(unsigned long) * 2;
+ pgprotval_t pr = pgprot_val(st->current_prot);
+
+ if (st->check_wx && (pr & _PAGE_RW) && !(pr & _PAGE_NX)) {
+ WARN_ONCE(1,
+ "x86/mm: Found insecure W+X mapping at address %p/%pS\n",
+ (void *)st->start_address,
+ (void *)st->start_address);
+ st->wx_pages += (st->current_address -
+ st->start_address) / PAGE_SIZE;
+ }
/*
* Now print the actual finished series
@@ -344,7 +356,8 @@ static void walk_pud_level(struct seq_file *m, struct pg_state *st, pgd_t addr,
#define pgd_none(a) pud_none(__pud(pgd_val(a)))
#endif
-void ptdump_walk_pgd_level(struct seq_file *m, pgd_t *pgd)
+static void ptdump_walk_pgd_level_core(struct seq_file *m, pgd_t *pgd,
+ bool checkwx)
{
#ifdef CONFIG_X86_64
pgd_t *start = (pgd_t *) &init_level4_pgt;
@@ -359,6 +372,10 @@ void ptdump_walk_pgd_level(struct seq_file *m, pgd_t *pgd)
st.to_dmesg = true;
}
+ st.check_wx = checkwx;
+ if (checkwx)
+ st.wx_pages = 0;
+
for (i = 0; i < PTRS_PER_PGD; i++) {
st.current_address = normalize_addr(i * PGD_LEVEL_MULT);
if (!pgd_none(*start)) {
@@ -378,8 +395,26 @@ void ptdump_walk_pgd_level(struct seq_file *m, pgd_t *pgd)
/* Flush out the last page */
st.current_address = normalize_addr(PTRS_PER_PGD*PGD_LEVEL_MULT);
note_page(m, &st, __pgprot(0), 0);
+ if (!checkwx)
+ return;
+ if (st.wx_pages)
+ pr_info("x86/mm: Checked W+X mappings: FAILED, %lu W+X pages found.\n",
+ st.wx_pages);
+ else
+ pr_info("x86/mm: Checked W+X mappings: passed, no W+X pages found.\n");
+}
+
+void ptdump_walk_pgd_level(struct seq_file *m, pgd_t *pgd)
+{
+ ptdump_walk_pgd_level_core(m, pgd, false);
}
+void ptdump_walk_pgd_level_checkwx(void)
+{
+ ptdump_walk_pgd_level_core(NULL, NULL, true);
+}
+
+#ifdef CONFIG_X86_PTDUMP
static int ptdump_show(struct seq_file *m, void *v)
{
ptdump_walk_pgd_level(m, NULL);
@@ -397,10 +432,13 @@ static const struct file_operations ptdump_fops = {
.llseek = seq_lseek,
.release = single_release,
};
+#endif
static int pt_dump_init(void)
{
+#ifdef CONFIG_X86_PTDUMP
struct dentry *pe;
+#endif
#ifdef CONFIG_X86_32
/* Not a compile-time constant on x86-32 */
@@ -412,10 +450,12 @@ static int pt_dump_init(void)
address_markers[FIXADDR_START_NR].start_address = FIXADDR_START;
#endif
+#ifdef CONFIG_X86_PTDUMP
pe = debugfs_create_file("kernel_page_tables", 0600, NULL, NULL,
&ptdump_fops);
if (!pe)
return -ENOMEM;
+#endif
return 0;
}
diff --git a/arch/x86/mm/init_32.c b/arch/x86/mm/init_32.c
index 7562f42..cb4ef3d 100644
--- a/arch/x86/mm/init_32.c
+++ b/arch/x86/mm/init_32.c
@@ -957,6 +957,8 @@ void mark_rodata_ro(void)
set_pages_ro(virt_to_page(start), size >> PAGE_SHIFT);
#endif
mark_nxdata_nx();
+ if (__supported_pte_mask & _PAGE_NX)
+ debug_checkwx();
}
#endif
diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index df48430..5ed62ef 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -1150,6 +1150,8 @@ void mark_rodata_ro(void)
free_init_pages("unused kernel",
(unsigned long) __va(__pa_symbol(rodata_end)),
(unsigned long) __va(__pa_symbol(_sdata)));
+
+ debug_checkwx();
}
#endif
--
2.1.0
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