Re: [PATCH] iopl.2: Changing description of permissions set per-process to per-thread

["Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>]

[CC += Thomas Gleixner]

Thomas G,

I expect that the small change at Thomas P proposes in this patch is
correct (i.e., iopl(2) operates per-thread, not per-process). I
remember that you touch the relevant kernel source file often. Perhaps
you are able to give a quick Ack?

Thanks,

Michael


On Sun, 24 May 2020 at 15:22, Thomas Piekarski
<t.piekarski@deloquencia.de> wrote:
>
> iopl is setting permissions for port-mapped I/O not per-process but only
> for threads and its children.
>
> See https://bugzilla.kernel.org/show_bug.cgi?id=205317
>
> Reported-by: victorm007@yahoo.com
> Signed-off-by: Thomas Piekarski <t.piekarski@deloquencia.de>
>
>
> ---
>
>
> 1. Test case if permissions are granted per-process or per-thread
>
> I took the opportunity to dig into PMIO and granting permissions with
> iopl and ioperm.
>
> Wrote the following code in which two threads are created and try to
> read some data with inb(). First thread (read_from_sleepy_child) is
> created before permissions are granted (but is delayed) and the second
> one (read_from_child) after that.
>
> If those permissions would be granted on process level should the first
> thread not succeed?
>
> I hope I did not make any mistake, applied threading well and can solve
> this issue as well as support the discussion at LKML.
>
> 2. Test Code
>
> #include <pthread.h>
> #include <stdio.h>
> #include <sys/io.h>
> #include <unistd.h>
>
> #define PORT 0x378 // lp0
>
> void *read_from_sleepy_child()
> {
>    sleep(3);
>
>    // The inb() will fail due to missing permissions and it'll segfault
>    // although permissions are acquired before threads are joined.
>    // When permissions are set per process this should work.
>    printf("Read anything from (sleepy) child thread (%x).\n", inb(PORT));
>
>    return NULL;
> }
>
> void *read_from_child()
> {
>    // The inb() will succeed due to permissions are inherited to
>    // childs after they got acquired with either iopl or ioperm
>    printf("Read anything from child thread (%x).\n", inb(PORT));
>
>    return NULL;
> }
>
> int main()
> {
>    pthread_t delayed_thread, thread;
>
>    pthread_create(&delayed_thread, NULL, read_from_sleepy_child, NULL);
>
>    iopl(3);
>    // ioperm(0, 0xFFFF, 1); // the same segfault
>
>    // The inb() will succeed due to being the main, default thread
>    // where permissions got acquired in first place
>    printf("Read anything from main thread (%x).\n", inb(PORT));
>
>    pthread_create(&thread, NULL, read_from_child, NULL);
>    pthread_join(delayed_thread, NULL);
>    pthread_join(thread, NULL);
>
>    return 0;
> }
>
>
> 3. Patch
>
>   man2/iopl.2 | 6 +++---
>   1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/man2/iopl.2 b/man2/iopl.2
> index e5b216a14..329095808 100644
> --- a/man2/iopl.2
> +++ b/man2/iopl.2
> @@ -39,7 +39,7 @@ iopl \- change I/O privilege level
>   .BI "int iopl(int " level );
>   .SH DESCRIPTION
>   .BR iopl ()
> -changes the I/O privilege level of the calling process,
> +changes the I/O privilege level of the calling thread,
>   as specified by the two least significant bits in
>   .IR level .
>   .PP
> @@ -50,7 +50,7 @@ Since these X servers require access to all 65536 I/O
> ports, the
>   call is not sufficient.
>   .PP
>   In addition to granting unrestricted I/O port access, running at a higher
> -I/O privilege level also allows the process to disable interrupts.
> +I/O privilege level also allows the thread to disable interrupts.
>   This will probably crash the system, and is not recommended.
>   .PP
>   Permissions are not inherited by the child process created by
> @@ -79,7 +79,7 @@ is greater than 3.
>   This call is unimplemented.
>   .TP
>   .B EPERM
> -The calling process has insufficient privilege to call
> +The calling thread has insufficient privilege to call
>   .BR iopl ();
>   the
>   .B CAP_SYS_RAWIO
> --
> 2.20.1



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/