Re: Buggy commit tracked to: "Re: [PATCH 2/9] iov_iter: move rw_copy_check_uvector() into lib/iov_iter.c"

David Hildenbrand
2020-10-22T12:42:40Z

On 22.10.20 14:18, Greg KH wrote:
> On Thu, Oct 22, 2020 at 12:48:05PM +0200, Greg KH wrote:
>> On Thu, Oct 22, 2020 at 11:36:40AM +0200, David Hildenbrand wrote:
>>> On 22.10.20 11:32, David Laight wrote:
>>>> From: David Hildenbrand
>>>>> Sent: 22 October 2020 10:25
>>>> ...
>>>>> ... especially because I recall that clang and gcc behave slightly
>>>>> differently:
>>>>>
>>>>> https://github.com/hjl-tools/x86-psABI/issues/2
>>>>>
>>>>> "Function args are different: narrow types are sign or zero extended to
>>>>> 32 bits, depending on their type. clang depends on this for incoming
>>>>> args, but gcc doesn't make that assumption. But both compilers do it
>>>>> when calling, so gcc code can call clang code.
>>>>
>>>> It really is best to use 'int' (or even 'long') for all numeric
>>>> arguments (and results) regardless of the domain of the value.
>>>>
>>>> Related, I've always worried about 'bool'....
>>>>
>>>>> The upper 32 bits of registers are always undefined garbage for types
>>>>> smaller than 64 bits."
>>>>
>>>> On x86-64 the high bits are zeroed by all 32bit loads.
>>>
>>> Yeah, but does not help here.
>>>
>>>
>>> My thinking: if the compiler that calls import_iovec() has garbage in
>>> the upper 32 bit
>>>
>>> a) gcc will zero it out and not rely on it being zero.
>>> b) clang will not zero it out, assuming it is zero.
>>>
>>> But
>>>
>>> a) will zero it out when calling the !inlined variant
>>> b) clang will zero it out when calling the !inlined variant
>>>
>>> When inlining, b) strikes. We access garbage. That would mean that we
>>> have calling code that's not generated by clang/gcc IIUC.
>>>
>>> We can test easily by changing the parameters instead of adding an "inline".
>>
>> Let me try that as well, as I seem to have a good reproducer, but it
>> takes a while to run...
>
> Ok, that didn't work.
>
> And I can't seem to "fix" this by adding noinline to patches further
> along in the patch series (because this commit's function is no longer
> present due to later patches.)

We might have the same issues with iovec_from_user() and friends now.

>
> Will keep digging...
>
> greg k-h
>

Might be worth to give this a try, just to see if it's related to
garbage in upper 32 bit and the way clang is handling it (might be a BUG
in clang, though):

diff --git a/include/linux/uio.h b/include/linux/uio.h
index 72d88566694e..7527298c6b56 100644
--- a/include/linux/uio.h
+++ b/include/linux/uio.h
@@ -267,7 +267,7 @@ size_t hash_and_copy_to_iter(const void *addr,
size_t bytes, void *hashp,
struct iov_iter *i);

struct iovec *iovec_from_user(const struct iovec __user *uvector,
- unsigned long nr_segs, unsigned long fast_segs,
+ unsigned nr_segs, unsigned fast_segs,
struct iovec *fast_iov, bool compat);
ssize_t import_iovec(int type, const struct iovec __user *uvec,
unsigned nr_segs, unsigned fast_segs, struct iovec **iovp,
diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 1635111c5bd2..58417f1916dc 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -1652,7 +1652,7 @@ const void *dup_iter(struct iov_iter *new, struct
iov_iter *old, gfp_t flags)
EXPORT_SYMBOL(dup_iter);

static int copy_compat_iovec_from_user(struct iovec *iov,
- const struct iovec __user *uvec, unsigned long nr_segs)
+ const struct iovec __user *uvec, unsigned nr_segs)
{
const struct compat_iovec __user *uiov =
(const struct compat_iovec __user *)uvec;
@@ -1684,7 +1684,7 @@ static int copy_compat_iovec_from_user(struct
iovec *iov,
}

static int copy_iovec_from_user(struct iovec *iov,
- const struct iovec __user *uvec, unsigned long nr_segs)
+ const struct iovec __user *uvec, unsigned nr_segs)
{
unsigned long seg;

@@ -1699,7 +1699,7 @@ static int copy_iovec_from_user(struct iovec *iov,
}

struct iovec *iovec_from_user(const struct iovec __user *uvec,
- unsigned long nr_segs, unsigned long fast_segs,
+ unsigned nr_segs, unsigned fast_segs,
struct iovec *fast_iov, bool compat)
{
struct iovec *iov = fast_iov;
@@ -1738,7 +1738,7 @@ ssize_t __import_iovec(int type, const struct
iovec __user *uvec,
struct iov_iter *i, bool compat)
{
ssize_t total_len = 0;
- unsigned long seg;
+ unsigned seg;
struct iovec *iov;

iov = iovec_from_user(uvec, nr_segs, fast_segs, *iovp, compat);

--
Thanks,

David / dhildenb