[PATCH v2 1/2] Fix undefined behaviour

[Grzegorz Uriasz <gorbak25@xxxxxxxxx>]

This patch fixes qemu crashes when passing through an IGD device to HVM guests 
under XEN. The problem is that on almost every laptop
reading the IGD ROM from SYSFS will fail, the reason for it is that the IGD rom 
is polymorphic and it modifies itself
during bootup - this results in an invalid rom image - the kernel checks 
whether the image is valid and when that's not the case
it won't allow userspace to read it. It seems that when the code was first 
written(xen_pt_load_rom.c) the kernel's back then didn't check
whether the rom is valid and just passed the contents to userspace - because of 
this qemu also tries to repair the rom later in the code.
When stating the rom the kernel isn't validating the rom contents so it is 
returning the proper size of the rom(32 4kb pages).

This results in undefined behaviour - pci_assign_dev_load_option_rom is 
creating a buffer and then writes the size of the buffer to a pointer.
In pci_assign_dev_load_option_rom under old kernels if the fstat would succeed 
then fread would also succeed - this means if the buffer
is allocated the size of the buffer will be set. On newer kernels this is not 
the case - on all laptops I've tested(spanning a few
generations of IGD) the fstat is successful and the buffer is allocated but the 
fread is failing - as the buffer is not deallocated
the function is returning a valid pointer without setting the size of the 
buffer for the caller. The caller of pci_assign_dev_load_option_rom
is holding the size of the buffer in an uninitialized variable and is just 
checking whether pci_assign_dev_load_option_rom returned a valid pointer.
This later results in cpu_physical_memory_write(0xc0000, 
result_of_pci_assign_dev_load_option_rom, unitialized_variable) which
depending on the compiler parameters, configure flags, etc... might crash qemu 
or might work - the xen 4.12 stable release with default configure
parameters works but changing the compiler options slightly(for instance by 
enabling qemu debug) results in qemu crashing ¯\_(;-;)_/¯

The only situation when the original pci_assign_dev_load_option_rom works is 
when the IDG is not the boot gpu - this won't happen on any laptop and
will be rare on desktops.

This patch deallocates the buffer and returns NULL if reading the VBIOS fails - 
the caller of the function then properly shuts down qemu.
The next patch in the series introduces a better method for getting the vbios 
so qemu does not exit when pci_assign_dev_load_option_rom fails -
this is the reason I've changed error_report to warn_report as whether a 
failure in pci_assign_dev_load_option_rom is fatal
depends on the caller.

Signed-off-by: Grzegorz Uriasz <gorbak25@xxxxxxxxx>
---
 hw/xen/xen_pt_load_rom.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/hw/xen/xen_pt_load_rom.c b/hw/xen/xen_pt_load_rom.c
index a50a80837e..9f100dc159 100644
--- a/hw/xen/xen_pt_load_rom.c
+++ b/hw/xen/xen_pt_load_rom.c
@@ -38,12 +38,12 @@ void *pci_assign_dev_load_option_rom(PCIDevice *dev,
     fp = fopen(rom_file, "r+");
     if (fp == NULL) {
         if (errno != ENOENT) {
-            error_report("pci-assign: Cannot open %s: %s", rom_file, 
strerror(errno));
+            warn_report("pci-assign: Cannot open %s: %s", rom_file, 
strerror(errno));
         }
         return NULL;
     }
     if (fstat(fileno(fp), &st) == -1) {
-        error_report("pci-assign: Cannot stat %s: %s", rom_file, 
strerror(errno));
+        warn_report("pci-assign: Cannot stat %s: %s", rom_file, 
strerror(errno));
         goto close_rom;
     }
 
@@ -59,10 +59,9 @@ void *pci_assign_dev_load_option_rom(PCIDevice *dev,
     memset(ptr, 0xff, st.st_size);
 
     if (!fread(ptr, 1, st.st_size, fp)) {
-        error_report("pci-assign: Cannot read from host %s", rom_file);
-        error_printf("Device option ROM contents are probably invalid "
-                     "(check dmesg).\nSkip option ROM probe with rombar=0, "
-                     "or load from file with romfile=\n");
+        warn_report("pci-assign: Cannot read from host %s", rom_file);
+        memory_region_unref(&dev->rom);
+        ptr = NULL;
         goto close_rom;
     }
 
-- 
2.26.1