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Subject[PATCH] x86, x86_64: Fix checks for userspace address limit
Date
hi,
there seems to be bug in the _copy_to_user and _copy_from_user
functions, not allowing access to the last user page.

Also I tried to decipher the inline assembly in __range_not_ok,
and it seems to work properly, but the macro comment seems to
be misleading.

wbr,
jirka

---
As shown in BZ 30352 (https://bugzilla.kernel.org/show_bug.cgi?id=30352)
there's an issue with reading last allowed page on x86_64.
The _copy_to_user and _copy_from_user functions use following
check for address limit:

if (buf + size >= limit)
fail
while it should be:

if (buf + size > limit)
fail
That's because the size represents the number of bytes being
read/write from/to buf address AND including the buf address.
So the copy function will actually never touch the limit
address even if "buf + size == limit".

Following program fails to use the last page as buffer
due to the wrong limit check.

---
#include <sys/mman.h>
#include <sys/socket.h>
#include <assert.h>
#define PAGE_SIZE (4096)
#define LAST_PAGE ((void*)(0x7fffffffe000))
int main()
{
int fds[2], err;
void * ptr = mmap(LAST_PAGE, PAGE_SIZE, PROT_READ | PROT_WRITE,
MAP_ANONYMOUS | MAP_PRIVATE | MAP_FIXED, -1, 0);
assert(ptr == LAST_PAGE);
err = socketpair(AF_LOCAL, SOCK_STREAM, 0, fds);
assert(err == 0);
err = send(fds[0], ptr, PAGE_SIZE, 0);
perror("send");
assert(err == PAGE_SIZE);
err = recv(fds[1], ptr, PAGE_SIZE, MSG_WAITALL);
perror("recv");
assert(err == PAGE_SIZE);
return 0;
}
---
Other place checking the addr limit is access_ok function,
which is working properly. There's just misleading comment
for the __range_not_ok macro.


Signed-off-by: Jiri Olsa <jolsa@redhat.com>
---
arch/x86/include/asm/uaccess.h | 2 +-
arch/x86/lib/copy_user_64.S | 4 ++--
2 files changed, 3 insertions(+), 3 deletions(-)
diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index abd3e0e..99f0ad7 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -42,7 +42,7 @@
* Returns 0 if the range is valid, nonzero otherwise.
*
* This is equivalent to the following test:
- * (u33)addr + (u33)size >= (u33)current->addr_limit.seg (u65 for x86_64)
+ * (u33)addr + (u33)size > (u33)current->addr_limit.seg (u65 for x86_64)
*
* This needs 33-bit (65-bit for x86_64) arithmetic. We have a carry...
*/
diff --git a/arch/x86/lib/copy_user_64.S b/arch/x86/lib/copy_user_64.S
index 99e4826..a73397f 100644
--- a/arch/x86/lib/copy_user_64.S
+++ b/arch/x86/lib/copy_user_64.S
@@ -72,7 +72,7 @@ ENTRY(_copy_to_user)
addq %rdx,%rcx
jc bad_to_user
cmpq TI_addr_limit(%rax),%rcx
- jae bad_to_user
+ ja bad_to_user
ALTERNATIVE_JUMP X86_FEATURE_REP_GOOD,copy_user_generic_unrolled,copy_user_generic_string
CFI_ENDPROC
ENDPROC(_copy_to_user)
@@ -85,7 +85,7 @@ ENTRY(_copy_from_user)
addq %rdx,%rcx
jc bad_from_user
cmpq TI_addr_limit(%rax),%rcx
- jae bad_from_user
+ ja bad_from_user
ALTERNATIVE_JUMP X86_FEATURE_REP_GOOD,copy_user_generic_unrolled,copy_user_generic_string
CFI_ENDPROC
ENDPROC(_copy_from_user)
--
1.7.1


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