[PATCH v4] x86/gart/kcore: Exclude GART aperture from kcore

[Kairui Song]

On machines where the GART aperture is mapped over physical RAM,
/proc/kcore contains the GART aperture range and reading it may lead
to kernel panic.

Vmcore used to have the same issue, until we fixed it in
commit 2a3e83c6f96c ("x86/gart: Exclude GART aperture from vmcore")',
leveraging existing hook infrastructure in vmcore to let /proc/vmcore
return zeroes when attempting to read the aperture region, and so it
won't read from the actual memory.

We apply the same workaround for kcore. First implement the same hook
infrastructure for kcore, then reuse the hook function introduced in
previous vmcore fix. Just with some minor adjustment, rename some
functions for more general usage, and simplify the hook infrastructure
a bit as there is no module usage yet.

Suggested-by: Baoquan He <bhe@xxxxxxxxxx>
Signed-off-by: Kairui Song <kasong@xxxxxxxxxx>

---

Update from V3:
- Reuse the approach in V2, as Jiri noticed V3 approach may fail
  some use case. It introduce overlapped region in kcore, and can't
  garenteen the read request will fall into the region we wanted.
- Improve some function naming suggested by Baoquan in V2.
- Simplify the hook registering and checking, we are not exporting the
  hook register function for now, no need to make it that complex.
- Simplify the commit message

Update from V2:
Instead of repeating the same hook infrastructure for kcore, introduce
a new kcore area type to avoid reading from, and let kcore always bypass
this kind of area.

Update from V1:
Fix a complie error when CONFIG_PROC_KCORE is not set

 arch/x86/kernel/aperture_64.c | 20 +++++++++++++-------
 fs/proc/kcore.c               | 32 ++++++++++++++++++++++++++++++++
 include/linux/kcore.h         |  3 +++
 3 files changed, 48 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kernel/aperture_64.c b/arch/x86/kernel/aperture_64.c
index 58176b56354e..c1319567b441 100644
--- a/arch/x86/kernel/aperture_64.c
+++ b/arch/x86/kernel/aperture_64.c
@@ -14,6 +14,7 @@
 #define pr_fmt(fmt) "AGP: " fmt
 
 #include <linux/kernel.h>
+#include <linux/kcore.h>
 #include <linux/types.h>
 #include <linux/init.h>
 #include <linux/memblock.h>
@@ -57,7 +58,7 @@ int fallback_aper_force __initdata;
 
 int fix_aperture __initdata = 1;
 
-#ifdef CONFIG_PROC_VMCORE
+#if defined(CONFIG_PROC_VMCORE) || defined(CONFIG_PROC_KCORE)
 /*
  * If the first kernel maps the aperture over e820 RAM, the kdump kernel will
  * use the same range because it will remain configured in the northbridge.
@@ -66,20 +67,25 @@ int fix_aperture __initdata = 1;
  */
 static unsigned long aperture_pfn_start, aperture_page_count;
 
-static int gart_oldmem_pfn_is_ram(unsigned long pfn)
+static int gart_mem_pfn_is_ram(unsigned long pfn)
 {
 	return likely((pfn < aperture_pfn_start) ||
 		      (pfn >= aperture_pfn_start + aperture_page_count));
 }
 
-static void exclude_from_vmcore(u64 aper_base, u32 aper_order)
+static void exclude_from_core(u64 aper_base, u32 aper_order)
 {
 	aperture_pfn_start = aper_base >> PAGE_SHIFT;
 	aperture_page_count = (32 * 1024 * 1024) << aper_order >> PAGE_SHIFT;
-	WARN_ON(register_oldmem_pfn_is_ram(&gart_oldmem_pfn_is_ram));
+#ifdef CONFIG_PROC_VMCORE
+	WARN_ON(register_oldmem_pfn_is_ram(&gart_mem_pfn_is_ram));
+#endif
+#ifdef CONFIG_PROC_KCORE
+	WARN_ON(register_mem_pfn_is_ram(&gart_mem_pfn_is_ram));
+#endif
 }
 #else
-static void exclude_from_vmcore(u64 aper_base, u32 aper_order)
+static void exclude_from_core(u64 aper_base, u32 aper_order)
 {
 }
 #endif
@@ -474,7 +480,7 @@ int __init gart_iommu_hole_init(void)
 			 * may have allocated the range over its e820 RAM
 			 * and fixed up the northbridge
 			 */
-			exclude_from_vmcore(last_aper_base, last_aper_order);
+			exclude_from_core(last_aper_base, last_aper_order);
 
 			return 1;
 		}
@@ -520,7 +526,7 @@ int __init gart_iommu_hole_init(void)
 	 * overlap with the first kernel's memory. We can't access the
 	 * range through vmcore even though it should be part of the dump.
 	 */
-	exclude_from_vmcore(aper_alloc, aper_order);
+	exclude_from_core(aper_alloc, aper_order);
 
 	/* Fix up the north bridges */
 	for (i = 0; i < amd_nb_bus_dev_ranges[i].dev_limit; i++) {
diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c
index bbcc185062bb..e51b324450d6 100644
--- a/fs/proc/kcore.c
+++ b/fs/proc/kcore.c
@@ -54,6 +54,33 @@ static LIST_HEAD(kclist_head);
 static DECLARE_RWSEM(kclist_lock);
 static int kcore_need_update = 1;
 
+/*
+ * Returns > 0 for RAM pages, 0 for non-RAM pages, < 0 on error
+ * Same as oldmem_pfn_is_ram in vmcore
+ */
+static int (*mem_pfn_is_ram)(unsigned long pfn);
+
+int register_mem_pfn_is_ram(int (*fn)(unsigned long pfn))
+{
+	if (mem_pfn_is_ram)
+		return -EBUSY;
+	mem_pfn_is_ram = fn;
+	return 0;
+}
+
+void unregister_mem_pfn_is_ram(void)
+{
+	mem_pfn_is_ram = NULL;
+}
+
+static int pfn_is_ram(unsigned long pfn)
+{
+	if (mem_pfn_is_ram)
+		return mem_pfn_is_ram(pfn);
+	else
+		return 1;
+}
+
 /* This doesn't grab kclist_lock, so it should only be used at init time. */
 void __init kclist_add(struct kcore_list *new, void *addr, size_t size,
 		       int type)
@@ -465,6 +492,11 @@ read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
 				goto out;
 			}
 			m = NULL;	/* skip the list anchor */
+		} else if (!pfn_is_ram(__pa(start) >> PAGE_SHIFT)) {
+			if (clear_user(buffer, tsz)) {
+				ret = -EFAULT;
+				goto out;
+			}
 		} else if (m->type == KCORE_VMALLOC) {
 			vread(buf, (char *)start, tsz);
 			/* we have to zero-fill user buffer even if no read */
diff --git a/include/linux/kcore.h b/include/linux/kcore.h
index 8c3f8c14eeaa..2130a35f883b 100644
--- a/include/linux/kcore.h
+++ b/include/linux/kcore.h
@@ -44,6 +44,9 @@ void kclist_add_remap(struct kcore_list *m, void *addr, void *vaddr, size_t sz)
 	m->vaddr = (unsigned long)vaddr;
 	kclist_add(m, addr, sz, KCORE_REMAP);
 }
+
+extern int register_mem_pfn_is_ram(int (*fn)(unsigned long pfn));
+extern void unregister_mem_pfn_is_ram(void);
 #else
 static inline
 void kclist_add(struct kcore_list *new, void *addr, size_t size, int type)
-- 
2.20.1