Re: [PATCH 1/1] swiotlb: Fix swiotlb_bounce() to do partial sync's correctly

[Dominique Martinet]

H Michael,

mhkelley58@xxxxxxxxx wrote on Tue, Mar 26, 2024 at 08:45:48PM -0700:
> From: Michael Kelley <mhklinux@xxxxxxxxxxx>
> 
> In current code, swiotlb_bounce() may do partial sync's correctly in
> some circumstances, but may incorrectly fail in other circumstances.
> The failure cases require both of these to be true:
> 
> 1) swiotlb_align_offset() returns a non-zero "offset" value
> 2) the tlb_addr of the partial sync area points into the first
> "offset" bytes of the _second_ or subsequent swiotlb slot allocated
> for the mapping
> 
> Code added in commit 868c9ddc182b ("swiotlb: add overflow checks
> to swiotlb_bounce") attempts to WARN on the invalid case where
> tlb_addr points into the first "offset" bytes of the _first_
> allocated slot. But there's no way for swiotlb_bounce() to distinguish
> the first slot from the second and subsequent slots, so the WARN
> can be triggered incorrectly when #2 above is true.
> 
> Related, current code calculates an adjustment to the orig_addr stored
> in the swiotlb slot. The adjustment compensates for the difference
> in the tlb_addr used for the partial sync vs. the tlb_addr for the full
> mapping. The adjustment is stored in the local variable tlb_offset.
> But when #1 and #2 above are true, it's valid for this adjustment to
> be negative. In such case the arithmetic to adjust orig_addr produces
> the wrong result due to tlb_offset being declared as unsigned.
> 
> Fix these problems by removing the over-constraining validations added
> in 868c9ddc182b. Change the declaration of tlb_offset to be signed
> instead of unsigned so the adjustment arithmetic works correctly.
> 
> Tested with a test-only hack to how swiotlb_tbl_map_single() calls
> swiotlb_bounce(). Instead of calling swiotlb_bounce() just once
> for the entire mapped area, do a loop with each iteration doing
> only a 128 byte partial sync until the entire mapped area is
> sync'ed. Then with swiotlb=force on the kernel boot line, run a
> variety of raw disk writes followed by read and verification of
> all bytes of the written data. The storage device has DMA
> min_align_mask set, and the writes are done with a variety of
> original buffer memory address alignments and overall buffer
> sizes. For many of the combinations, current code triggers the
> WARN statements, or the data verification fails. With the fixes,
> no WARNs occur and all verifications pass.

Thanks for the detailed analysis & test, and sorry I didn't reply to
your mail last week.
For everyone's benefit I'll reply here (question was "what were the two
bad commits for"), it's that the previous rework introduced a regression
with caamjr on unaligned mappings that I had reported here:
https://lore.kernel.org/lkml/YL7XXNOnbaDgmTB9@xxxxxxxxxxxxxxxxx/#t

(took me a while to find the threads, Link: tags have been sparse...)

Unfortunately that was ages ago so I don't really remember exactly on
which device that was reproduced.. Given the Cc I'm sure Lukas had hit
it on the MNT reform (i.MX8MQ), but I did say I tested it so I probably
could reproduce on my i.MX8MP?
I'll try to give a try at reproducing the old bug, and if I do test your
fix over next week.


> 
> Fixes: 5f89468e2f06 ("swiotlb: manipulate orig_addr when tlb_addr has offset")
> Fixes: 868c9ddc182b ("swiotlb: add overflow checks to swiotlb_bounce")
> Signed-off-by: Michael Kelley <mhklinux@xxxxxxxxxxx>
> ---
> This patch is built against the 6.9-rc1 tree plus Petr Tesarik's
> "swiotlb: extend buffer pre-padding to alloc_align_mask" patch
> that's in-flight.
> 
>  kernel/dma/swiotlb.c | 30 +++++++++++++-----------------
>  1 file changed, 13 insertions(+), 17 deletions(-)
> 
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index d7a8cb93ef2d..d57c8837c813 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -863,27 +863,23 @@ static void swiotlb_bounce(struct device *dev, phys_addr_t tlb_addr, size_t size
>  	size_t alloc_size = mem->slots[index].alloc_size;
>  	unsigned long pfn = PFN_DOWN(orig_addr);
>  	unsigned char *vaddr = mem->vaddr + tlb_addr - mem->start;
> -	unsigned int tlb_offset, orig_addr_offset;
> +	int tlb_offset;
>  
>  	if (orig_addr == INVALID_PHYS_ADDR)
>  		return;
>  
> -	tlb_offset = tlb_addr & (IO_TLB_SIZE - 1);
> -	orig_addr_offset = swiotlb_align_offset(dev, 0, orig_addr);
> -	if (tlb_offset < orig_addr_offset) {
> -		dev_WARN_ONCE(dev, 1,
> -			"Access before mapping start detected. orig offset %u, requested offset %u.\n",
> -			orig_addr_offset, tlb_offset);
> -		return;
> -	}
> -
> -	tlb_offset -= orig_addr_offset;
> -	if (tlb_offset > alloc_size) {
> -		dev_WARN_ONCE(dev, 1,
> -			"Buffer overflow detected. Allocation size: %zu. Mapping size: %zu+%u.\n",
> -			alloc_size, size, tlb_offset);
> -		return;
> -	}
> +	/*
> +	 * It's valid for tlb_offset to be negative. This can happen when the
> +	 * "offset" returned by swiotlb_align_offset() is non-zero, and the
> +	 * tlb_addr is pointing within the first "offset" bytes of the second
> +	 * or subsequent slots of the allocated swiotlb area. While it's not
> +	 * valid for tlb_addr to be pointing within the first "offset" bytes
> +	 * of the first slot, there's no way to check for such an error since
> +	 * this function can't distinguish the first slot from the second and
> +	 * subsequent slots.
> +	 */
> +	tlb_offset = (tlb_addr & (IO_TLB_SIZE - 1)) -
> +		     swiotlb_align_offset(dev, 0, orig_addr);
>  
>  	orig_addr += tlb_offset;
>  	alloc_size -= tlb_offset;

-- 
Dominique Martinet | Asmadeus