[PATCH v2] fbdev: Prevent probing generic drivers if a FB is already registered
Javier Martinez Canillas
javierm at redhat.com
Tue Nov 16 13:49:21 UTC 2021
On 11/16/21 11:01, Javier Martinez Canillas wrote:
> Hello Geert,
>
> On 11/16/21 10:43, Geert Uytterhoeven wrote:
>
> [snip]
>
>>>
>>> So this is already a fragile solution and $SUBJECT doesn't make things worse
>>> IMO. Since not having something like this can lead to issues as reported by:
>>>
>>> https://lore.kernel.org/all/20211110200253.rfudkt3edbd3nsyj@lahvuun/
>>>
>>> We could probably do some smarter here by providing a function that checks
>>> if the registered fbdev drivers matches the aperture base. But I'm unsure
>>> if that's worth it. After all, fbdev drivers are likely to be disabled by
>>> most distros soon now that we have the simpledrm driver.
>>
>> Checking the aperture base is what was done in all other cases of
>> preventing generic (fbdev) drivers from stepping on specific drivers'
>> toes...
>>
>
> Ok, I can re-spin the patch checking if the aperture ranges overlap. There's
> an apertures_overlap() function in drivers/video/fbdev/core/fbmem.c that can
> be exported for fbdev drivers to use.
>
So I tried the following patch [0]. But when testing on a VM, the efifb driver
is probed even after the virtio_gpu driver has already been probed. Being a DRM
driver, it doesn't use the fbdev infra and AFAIU doesn't reserve any apertures.
When the {efi,simple}fb drivers check if there's an aperture already reserved
using the fb_aperture_registered() helper, this just returns false even when a
driver for the same hardware was already registered. The kernel log says:
[ 0.891512] checking generic (0 0) vs hw (c0000000 1d5000)
That is because when DRM_FBDEV_EMULATION=y, the virtio_gpu driver registers an
fbdev but without any aperture set.
I discussed this with Thomas and even though $SUBJECT is just a workaround, it
seems that is the best we can do as an heuristic to prevent the generic fbdev
drivers to be probed after a native DRM driver.
[0]:
>From d962c20bc9fd90c2525d79b69e632d99e8050fc5 Mon Sep 17 00:00:00 2001
From: Javier Martinez Canillas <javierm at redhat.com>
Date: Thu, 11 Nov 2021 00:55:06 +0100
Subject: [PATCH v4] fbdev: Prevent probing generic drivers if a FB is already
registered
The efifb and simplefb drivers just render to a pre-allocated frame buffer
and rely on the display hardware being initialized before the kernel boots.
But if another driver already probed correctly and registered a fbdev, the
generic drivers shouldn't be probed since an actual driver for the display
hardware is already present.
This is more likely to occur after commit d391c5827107 ("drivers/firmware:
move x86 Generic System Framebuffers support") since the "efi-framebuffer"
and "simple-framebuffer" platform devices are registered at a later time.
Link: https://lore.kernel.org/r/20211110200253.rfudkt3edbd3nsyj@lahvuun/
Fixes: d391c5827107 ("drivers/firmware: move x86 Generic System Framebuffers support")
Reported-by: Ilya Trukhanov <lahvuun at gmail.com>
Cc: <stable at vger.kernel.org> # 5.15.x
Signed-off-by: Javier Martinez Canillas <javierm at redhat.com>
---
Changes in v4:
- Only fail to probe if a registered fbdev has overlapping aperture (Geert).
Changes in v3:
- Cc <stable at vger.kernel.org> since a Fixes: tag is not enough (gregkh).
Changes in v2:
- Add a Link: tag with a reference to the bug report (Thorsten Leemhuis).
- Add a comment explaining why the probe fails earlier (Daniel Vetter).
- Add a Fixes: tag for stable to pick the fix (Daniel Vetter).
- Add Daniel Vetter's Reviewed-by: tag.
- Improve the commit message and mention the culprit commit
drivers/video/fbdev/core/fbmem.c | 16 ++++++++++++++++
drivers/video/fbdev/efifb.c | 11 +++++++++++
drivers/video/fbdev/simplefb.c | 11 +++++++++++
include/linux/fb.h | 1 +
4 files changed, 39 insertions(+)
diff --git drivers/video/fbdev/core/fbmem.c drivers/video/fbdev/core/fbmem.c
index 826175ad88a2..9906b83132cb 100644
--- drivers/video/fbdev/core/fbmem.c
+++ drivers/video/fbdev/core/fbmem.c
@@ -1546,6 +1546,22 @@ static bool fb_do_apertures_overlap(struct apertures_struct *gena,
return false;
}
+bool fb_aperture_registered(struct apertures_struct *a)
+{
+ int i;
+
+ for_each_registered_fb(i) {
+ struct apertures_struct *gen_aper;
+
+ gen_aper = registered_fb[i]->apertures;
+ if (fb_do_apertures_overlap(gen_aper, a))
+ return true;
+ }
+
+ return false;
+}
+EXPORT_SYMBOL(fb_aperture_registered);
+
static void do_unregister_framebuffer(struct fb_info *fb_info);
#define VGA_FB_PHYS 0xA0000
diff --git drivers/video/fbdev/efifb.c drivers/video/fbdev/efifb.c
index edca3703b964..1ad6698b2e05 100644
--- drivers/video/fbdev/efifb.c
+++ drivers/video/fbdev/efifb.c
@@ -457,6 +457,17 @@ static int efifb_probe(struct platform_device *dev)
info->apertures->ranges[0].base = efifb_fix.smem_start;
info->apertures->ranges[0].size = size_remap;
+ /*
+ * Generic drivers must not be registered if a framebuffer exists.
+ * If a native driver was probed, the display hardware was already
+ * taken and attempting to use the system framebuffer is dangerous.
+ */
+ if (fb_aperture_registered(info->apertures)) {
+ dev_err(&dev->dev,
+ "efifb: a framebuffer is already registered\n");
+ return -EINVAL;
+ }
+
if (efi_enabled(EFI_MEMMAP) &&
!efi_mem_desc_lookup(efifb_fix.smem_start, &md)) {
if ((efifb_fix.smem_start + efifb_fix.smem_len) >
diff --git drivers/video/fbdev/simplefb.c drivers/video/fbdev/simplefb.c
index 62f0ded70681..3ad0f538ca91 100644
--- drivers/video/fbdev/simplefb.c
+++ drivers/video/fbdev/simplefb.c
@@ -456,6 +456,17 @@ static int simplefb_probe(struct platform_device *pdev)
info->apertures->ranges[0].base = info->fix.smem_start;
info->apertures->ranges[0].size = info->fix.smem_len;
+ /*
+ * Generic drivers must not be registered if a framebuffer exists.
+ * If a native driver was probed, the display hardware was already
+ * taken and attempting to use the system framebuffer is dangerous.
+ */
+ if (fb_aperture_registered(info->apertures)) {
+ dev_err(&pdev->dev,
+ "simplefb: a framebuffer is already registered\n");
+ return -EINVAL;
+ }
+
info->fbops = &simplefb_ops;
info->flags = FBINFO_DEFAULT | FBINFO_MISC_FIRMWARE;
info->screen_base = ioremap_wc(info->fix.smem_start,
diff --git include/linux/fb.h include/linux/fb.h
index 6f3db99ab990..f1fbdb39932b 100644
--- include/linux/fb.h
+++ include/linux/fb.h
@@ -604,6 +604,7 @@ extern ssize_t fb_sys_write(struct fb_info *info, const char __user *buf,
size_t count, loff_t *ppos);
/* drivers/video/fbmem.c */
+extern bool fb_aperture_registered(struct apertures_struct *a);
extern int register_framebuffer(struct fb_info *fb_info);
extern void unregister_framebuffer(struct fb_info *fb_info);
extern int remove_conflicting_pci_framebuffers(struct pci_dev *pdev,
--
2.33.1
Best regards,
--
Javier Martinez Canillas
Linux Engineering
Red Hat
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