Re: [PATCH] fix swiotlb panic on Xen

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

> As the person who first found this and then confirmed this fixes a bug:
> 
> Tested-by: Elliott Mitchell <ehem+xen@xxxxxxx>

Thank you!!

I changed the title and added the various tags and will put it in
linux-next later this week.

>From a1eb2768bf5954d25aa0f0136b38f0aa5d92d984 Mon Sep 17 00:00:00 2001
From: Stefano Stabellini <stefano.stabellini@xxxxxxxxxx>
Date: Mon, 26 Oct 2020 17:02:14 -0700
Subject: [PATCH] swiotlb: fix "x86: Don't panic if can not alloc buffer for
 swiotlb"

kernel/dma/swiotlb.c:swiotlb_init gets called first and tries to
allocate a buffer for the swiotlb. It does so by calling

  memblock_alloc_low(PAGE_ALIGN(bytes), PAGE_SIZE);

If the allocation must fail, no_iotlb_memory is set.

Later during initialization swiotlb-xen comes in
(drivers/xen/swiotlb-xen.c:xen_swiotlb_init) and given that io_tlb_start
is != 0, it thinks the memory is ready to use when actually it is not.

When the swiotlb is actually needed, swiotlb_tbl_map_single gets called
and since no_iotlb_memory is set the kernel panics.

Instead, if swiotlb-xen.c:xen_swiotlb_init knew the swiotlb hadn't been
initialized, it would do the initialization itself, which might still
succeed.

Fix the panic by setting io_tlb_start to 0 on swiotlb initialization
failure, and also by setting no_iotlb_memory to false on swiotlb
initialization success.

Fixes: ac2cbab21f31 ("x86: Don't panic if can not alloc buffer for swiotlb")

Reported-by: Elliott Mitchell <ehem+xen@xxxxxxx>
Tested-by: Elliott Mitchell <ehem+xen@xxxxxxx>
Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxx>
Reviewed-by: Christoph Hellwig <hch@xxxxxx>
CC: stable@xxxxxxxxxxxxxxx
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
---
 kernel/dma/swiotlb.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
index 465a567678d9..e08cac39c0ba 100644
--- a/kernel/dma/swiotlb.c
+++ b/kernel/dma/swiotlb.c
@@ -229,6 +229,7 @@ int __init swiotlb_init_with_tbl(char *tlb, unsigned long 
nslabs, int verbose)
                io_tlb_orig_addr[i] = INVALID_PHYS_ADDR;
        }
        io_tlb_index = 0;
+       no_iotlb_memory = false;
 
        if (verbose)
                swiotlb_print_info();
@@ -260,9 +261,11 @@ swiotlb_init(int verbose)
        if (vstart && !swiotlb_init_with_tbl(vstart, io_tlb_nslabs, verbose))
                return;
 
-       if (io_tlb_start)
+       if (io_tlb_start) {
                memblock_free_early(io_tlb_start,
                                    PAGE_ALIGN(io_tlb_nslabs << IO_TLB_SHIFT));
+               io_tlb_start = 0;
+       }
        pr_warn("Cannot allocate buffer");
        no_iotlb_memory = true;
 }
@@ -360,6 +363,7 @@ swiotlb_late_init_with_tbl(char *tlb, unsigned long nslabs)
                io_tlb_orig_addr[i] = INVALID_PHYS_ADDR;
        }
        io_tlb_index = 0;
+       no_iotlb_memory = false;
 
        swiotlb_print_info();
 
-- 
2.13.6