[xen staging] x86/debug: fix page-overflow bug in dbg_rw_guest_mem

[patchbot@xxxxxxx]

commit 9dc687f155a57216b83b17f9cde55dd43e06b0cd
Author:     Tamas K Lengyel <tamas@xxxxxxxxxxxxx>
AuthorDate: Fri Jan 29 20:59:53 2021 -0500
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Sat Jan 30 03:21:33 2021 +0000

    x86/debug: fix page-overflow bug in dbg_rw_guest_mem
    
    When using gdbsx dbg_rw_guest_mem is used to read/write guest memory. When 
the
    buffer being accessed is on a page-boundary, the next page needs to be 
grabbed
    to access the correct memory for the buffer's overflown parts. While
    dbg_rw_guest_mem has logic to handle that, it broke with 229492e210a. 
Instead
    of grabbing the next page the code right now is looping back to the
    start of the first page. This results in errors like the following while 
trying
    to use gdb with Linux' lx-dmesg:
    
    [    0.114457] PM: hibernation: Registered nosave memory: [mem
    0xfdfff000-0xffffffff]
    [    0.114460] [mem 0x90000000-0xfbffffff] available for PCI demem 0
    [    0.114462] f]f]
    Python Exception <class 'ValueError'> embedded null character:
    Error occurred in Python: embedded null character
    
    Fixing this bug by taking the variable assignment outside the loop.
    
    Fixes: 229492e210a ("x86/debugger: use copy_to/from_guest() in 
dbg_rw_guest_mem()")
    Signed-off-by: Tamas K Lengyel <tamas@xxxxxxxxxxxxx>
    Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
 xen/arch/x86/debug.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/debug.c b/xen/arch/x86/debug.c
index 4356039ed2..f32d4b0bcc 100644
--- a/xen/arch/x86/debug.c
+++ b/xen/arch/x86/debug.c
@@ -112,10 +112,11 @@ static unsigned int dbg_rw_guest_mem(struct domain *dp, 
void * __user gaddr,
                                      void * __user buf, unsigned int len,
                                      bool toaddr, uint64_t pgd3)
 {
+    unsigned long addr = (unsigned long)gaddr;
+
     while ( len > 0 )
     {
         char *va;
-        unsigned long addr = (unsigned long)gaddr;
         mfn_t mfn;
         gfn_t gfn = INVALID_GFN;
         unsigned long pagecnt;
--
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