From c8d10ab68534a651da2eb7a8bfff5ab04eb957cf Mon Sep 17 00:00:00 2001
From: John Hubbard <jhubbard@nvidia.com>
Date: Thu, 21 May 2020 21:15:05 -0700
Subject: video: fbdev: fix error handling for get_user_pages_fast()
Git-commit: e2e1c7bda4445363027a4ceed1cc3a8ccc2472a9
Patch-mainline: v5.9-rc1
References: jsc#SLE-12680, jsc#SLE-12880, jsc#SLE-12882, jsc#SLE-12883, jsc#SLE-13496, jsc#SLE-15322
Dealing with the return value of get_user_pages*() variants has a few
classic pitfalls, and this driver found one of them: the return value
might be zero, positive, or -errno. And if positive, it might be fewer
pages than were requested. And if fewer pages than requested, then
the caller should return (via put_page()) the pages that *were*
pinned.
This driver was doing that *except* that it had a problem with the
-errno case, which was being stored in an unsigned int, and which
would case an interesting mess if it ever happened: nr_pages would be
interpreted as a spectacularly huge unsigned value, rather than a
small negative value. Also, it was unnecessarily overriding a
potentially informative -errno, with -EINVAL, in some cases.
Instead: clamp the nr_pages to zero or positive, so that the error
handling works. And return the -errno value from get_user_pages*(),
unchanged, if we get one. And explain this with comments, seeing as
how it is error-prone.
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Gustavo A. R. Silva <gustavo@embeddedor.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-fbdev@vger.kernel.org
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200522041506.39638-2-jhubbard@nvidia.com
Signed-off-by: Patrik Jakobsson <pjakobsson@suse.de>
---
drivers/video/fbdev/pvr2fb.c | 18 ++++++++++++++++--
1 file changed, 16 insertions(+), 2 deletions(-)
diff --git a/drivers/video/fbdev/pvr2fb.c b/drivers/video/fbdev/pvr2fb.c
index f18d457175d9..ceb6ef590597 100644
--- a/drivers/video/fbdev/pvr2fb.c
+++ b/drivers/video/fbdev/pvr2fb.c
@@ -654,8 +654,22 @@ static ssize_t pvr2fb_write(struct fb_info *info, const char *buf,
ret = get_user_pages_fast((unsigned long)buf, nr_pages, FOLL_WRITE, pages);
if (ret < nr_pages) {
- nr_pages = ret;
- ret = -EINVAL;
+ if (ret < 0) {
+ /*
+ * Clamp the unsigned nr_pages to zero so that the
+ * error handling works. And leave ret at whatever
+ * -errno value was returned from GUP.
+ */
+ nr_pages = 0;
+ } else {
+ nr_pages = ret;
+ /*
+ * Use -EINVAL to represent a mildly desperate guess at
+ * why we got fewer pages (maybe even zero pages) than
+ * requested.
+ */
+ ret = -EINVAL;
+ }
goto out_unmap;
}
--
2.29.2