[RFC] kcore:change kcore_read to make sure the kernel read is safe

[yalin wang]

This change kcore_read() to use __copy_from_user_inatomic() to
copy data from kernel address, because kern_addr_valid() just make sure
page table is valid during call it, whne it return, the page table may
change, for example, like set_fixmap() function will change kernel page
table, then maybe trigger kernel crash if encounter this unluckily.

Signed-off-by: yalin wang <yalin.wang2010@xxxxxxxxx>
---
 fs/proc/kcore.c | 30 ++++++++++++++++++++++++------
 1 file changed, 24 insertions(+), 6 deletions(-)

diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c
index 92e6726..b085fde 100644
--- a/fs/proc/kcore.c
+++ b/fs/proc/kcore.c
@@ -86,8 +86,8 @@ static size_t get_kcore_size(int *nphdr, size_t *elf_buflen)
 			size = try;
 		*nphdr = *nphdr + 1;
 	}
-	*elf_buflen =	sizeof(struct elfhdr) + 
-			(*nphdr + 2)*sizeof(struct elf_phdr) + 
+	*elf_buflen =	sizeof(struct elfhdr) +
+			(*nphdr + 2)*sizeof(struct elf_phdr) +
 			3 * ((sizeof(struct elf_note)) +
 			     roundup(sizeof(CORE_STR), 4)) +
 			roundup(sizeof(struct elf_prstatus), 4) +
@@ -435,6 +435,7 @@ read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
 	size_t elf_buflen;
 	int nphdr;
 	unsigned long start;
+	unsigned long page = 0;
 
 	read_lock(&kclist_lock);
 	size = get_kcore_size(&nphdr, &elf_buflen);
@@ -485,7 +486,7 @@ read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
 	start = kc_offset_to_vaddr(*fpos - elf_buflen);
 	if ((tsz = (PAGE_SIZE - (start & ~PAGE_MASK))) > buflen)
 		tsz = buflen;
-		
+
 	while (buflen) {
 		struct kcore_list *m;
 
@@ -515,15 +516,32 @@ read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
 		} else {
 			if (kern_addr_valid(start)) {
 				unsigned long n;
+				mm_segment_t old_fs = get_fs();
+
+				if (page == 0) {
+					page = __get_free_page(GFP_KERNEL);
+					if (page == 0)
+						return -ENOMEM;
 
-				n = copy_to_user(buffer, (char *)start, tsz);
+				}
+				set_fs(KERNEL_DS);
+				pagefault_disable();
+				n = __copy_from_user_inatomic((void *)page,
+					(__force const void __user *)start,
+					tsz);
+				pagefault_enable();
+				set_fs(old_fs);
+				if (n)
+					memset((void *)page + tsz - n, 0, n);
+
+				n = copy_to_user(buffer, (char *)page, tsz);
 				/*
 				 * We cannot distinguish between fault on source
 				 * and fault on destination. When this happens
 				 * we clear too and hope it will trigger the
 				 * EFAULT again.
 				 */
-				if (n) { 
+				if (n) {
 					if (clear_user(buffer + tsz - n,
 								n))
 						return -EFAULT;
@@ -540,7 +558,7 @@ read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
 		start += tsz;
 		tsz = (buflen > PAGE_SIZE ? PAGE_SIZE : buflen);
 	}
-
+	free_page(page);
 	return acc;
 }
 
-- 
1.9.1

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