Re: [PATCH v2 3/3] powerpc/kexec_file: Prevent kexec range truncation

[Sourabh Jain <sourabhjain@linux.ibm.com>]

On 29/07/26 11:56, Jinjie Ruan wrote:
>
> 在 2026/7/29 12:43, Sourabh Jain 写道:
>>
>> On 29/07/26 06:59, Jinjie Ruan wrote:
>>> Sashiko AI review pointed out the following issue.
>>>
>>> The __merge_memory_ranges() function incorrectly handles overlapping
>>> memory ranges when merging them. Although sort_memory_ranges() sorts all
>>> ranges by their start address in ascending order beforehand, the merge
>>> logic remains defective in two ways:
>>>
>>> 1. It compares the current range's start against the previous element
>>> (i-1)
>>>      instead of the running target index (idx)
>>>
>>> 2. It unconditionally overwrites 'ranges[idx].end' with 'ranges[i].end'.
>>>
>>> This logic flaw leads to critical memory truncation when a larger memory
>>> range completely subsumes subsequent smaller ranges.
>>>
>>> For example, consider a sorted input array with three ranges:
>>>     Range A (idx=0): [0x1000 - 0x9000]
>>>     Range B (i=1):   [0x2000 - 0x5000] (completely inside Range A)
>>>     Range C (i=2):   [0x6000 - 0x8000] (completely inside Range A)
>>>
>>> 1. When i=1 (Range B):
>>>      ranges[1].start (0x2000) <= ranges[0].end + 1 (0x9001) is TRUE.
>>>      The code executes: ranges[0].end = ranges[1].end, which erroneously
>>>      shrinks Range A's end from 0x9000 down to 0x5000.
>>>
>>> 2. When i=2 (Range C):
>>>      ranges[2].start (0x6000) <= ranges[1].end + 1 (0x5001) is FALSE.
>>>      The code falls into the else block, creating a broken new range.
>>>
>>> As a result, valid memory fragments [0x5001 - 0x5fff] and [0x8001 -
>>> 0x9000]
>>> are completely lost from the kexec exclude lists, potentially allowing
>>> the crash kernel to overwrite active memory, causing data corruption
>>> or crashes.
>>>
>>> Fix this by ensuring the start of the current range is compared
>>> against the
>>> end of the active merged range (idx), and use max() to safely prevent the
>>> outer boundary from being truncated.
>>>
>>> Cc: Sourabh Jain <sourabhjain@linux.ibm.com>
>>> Cc: Hari Bathini <hbathini@linux.ibm.com>
>>> Cc: Michael Ellerman <mpe@ellerman.id.au>
>>> Cc: stable@vger.kernel.org
>>> Fixes: 180adfc532a8 ("powerpc/kexec_file: Add helper functions for
>>> getting memory ranges")
>>> Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
>>> ---
>>>    arch/powerpc/kexec/ranges.c | 12 +++++-------
>>>    1 file changed, 5 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/arch/powerpc/kexec/ranges.c b/arch/powerpc/kexec/ranges.c
>>> index e5fea23b191b..539061d14a77 100644
>>> --- a/arch/powerpc/kexec/ranges.c
>>> +++ b/arch/powerpc/kexec/ranges.c
>>> @@ -21,6 +21,7 @@
>>>    #include <linux/of.h>
>>>    #include <linux/slab.h>
>>>    #include <linux/memblock.h>
>>> +#include <linux/minmax.h>
>>>    #include <linux/crash_core.h>
>>>    #include <asm/sections.h>
>>>    #include <asm/kexec_ranges.h>
>>> @@ -105,19 +106,16 @@ static void __merge_memory_ranges(struct
>>> crash_mem *mem_rngs)
>>>        struct range *ranges;
>>>        int i, idx;
>>>    -    if (!mem_rngs)
>>> +    if (!mem_rngs || mem_rngs->nr_ranges <= 1)
>>>            return;
>> Although the below loop handles this but it is good to return early when
>> there is
>> only range.
>>
>>>          idx = 0;
>>> -    ranges = &(mem_rngs->ranges[0]);
>>> +    ranges = mem_rngs->ranges;
>>>        for (i = 1; i < mem_rngs->nr_ranges; i++) {
>>> -        if (ranges[i].start <= (ranges[i-1].end + 1))
>>> -            ranges[idx].end = ranges[i].end;
>>> +        if (ranges[i].start <= (ranges[idx].end + 1))
>>> +            ranges[idx].end = max(ranges[idx].end, ranges[i].end);
>> Yeah this changes is needed.
>>
>>>            else {
>>>                idx++;
>>> -            if (i == idx)
>>> -                continue;
>> Do we really need to remove the above condition?
>>
>> Isn't this condition is helpful till we find an overlap?
> Hi Sourabh,
>
> I believe there is no functional change here. The else branch itself
> already indicates that there will be no overlap this time, so we can
> safely use ranges[i] as the next memory region.

I agree that keeping or removing it doesn't change the functionality.

My point is that it helps avoid copying a range onto itself. For example:

Range A (idx = 0): [0x1000 - 0x2000]
Range B (idx = 1): [0x3000 - 0x5000]
Range C (idx = 2): [0x6000 - 0x8000]

For all three ranges,  idx = i = x and kernel will do:
ranges[x] = ranges[x];

This self-assignment can be avoided if we keep the condition instead of
removing it. Isn't it?

- Sourabh Jain


> Whether we remove it or keep it is fine.
>
>> - Sourabh Jain
>>
>>> -
>>>                ranges[idx] = ranges[i];
>>>            }
>>>        }